cmd/go: "test timed out after ... running tests: ..." attached to one of many subtests in JSON mode, can be less visible compared to non-JSON output

Consider the output from go test without -json flag in a situation where a test timeout happens while multiple subtests are still running (as happened in #74279):

$ go version
go version go1.25rc1 darwin/arm64

# in golang.org/x/tools/gopls/internal/test/marker package
$ go test -count=1 -timeout=20s ./... 
panic: test timed out after 20s
	running tests:
		Test/codeaction/fill_struct_resolve.txt (1s)
		Test/codeaction/issue64558.txt (0s)
		Test/codeaction/issue70268.txt (0s)
		Test/codeaction/moveparam.txt (1s)
		Test/codeaction/moveparam_issue70599.txt (1s)
		Test/codeaction/removeparam_formatting.txt (1s)
		Test/codeaction/removeparam_funcvalue.txt (1s)
		Test/codeaction/removeparam_imports.txt (1s)
		Test/codeaction/removeparam_resolve.txt (1s)
		Test/codeaction/splitlines.txt (2s)

goroutine 273028 [running]:
testing.(*M).startAlarm.func1()
	/usr/local/go/src/testing/testing.go:2679 +0x2b0
created by time.goFunc
	/usr/local/go/src/time/sleep.go:215 +0x38

goroutine 1 [chan receive]:
testing.(*T).Run(0x140003881c0, {0x104c4f030?, 0x14000045af8?}, 0x1052171d0)
	/usr/local/go/src/testing/testing.go:2002 +0x378
testing.runTests.func1(0x140003881c0)
	/usr/local/go/src/testing/testing.go:2474 +0x38
testing.tRunner(0x140003881c0, 0x14000045c28)
	/usr/local/go/src/testing/testing.go:1931 +0xc8
testing.runTests(0x140003c3a70, {0x105a54ec0, 0x1, 0x1}, {0x140004989a8?, 0x140004989d8?, 0x105a95520?})
	/usr/local/go/src/testing/testing.go:2472 +0x3b8
testing.(*M).Run(0x140003961e0)
	/usr/local/go/src/testing/testing.go:2334 +0x530
golang.org/x/tools/gopls/internal/test/marker.TestMain(0x140003961e0)
	/Users/gopher/go/src/golang.org/x/tools/gopls/internal/test/marker/marker_test.go:63 +0x54
main.main()
	_testmain.go:47 +0x88

goroutine 67 [chan receive]:
golang.org/x/tools/gopls/internal/lsprpc.(*streamServer).ServeStream(0x140000be2a0, {0x10522e850, 0x14007384b10}, {0x105233188, 0x1400758c140})
[…]
goroutine 273488 [runnable]:
golang.org/x/tools/gopls/internal/cache.execActions.func1()
	/Users/gopher/go/src/golang.org/x/tools/gopls/internal/cache/analysis.go:894
created by golang.org/x/tools/gopls/internal/cache.execActions in goroutine 273185
	/Users/gopher/go/src/golang.org/x/tools/gopls/internal/cache/analysis.go:894 +0x50
FAIL	golang.org/x/tools/gopls/internal/test/marker	20.916s
FAIL
$ echo $?
1

Compare with the output of go test in the same situation, with -json flag:

$ go test -json -count=1 -timeout=20s ./...
[…]
{"Time":"2025-06-20T12:45:40.364095-04:00","Action":"run","Package":"golang.org/x/tools/gopls/internal/test/marker","Test":"Test/codeaction/removeparam_resolve.txt"}
[…]
{"Time":"2025-06-20T12:45:40.364106-04:00","Action":"pause","Package":"golang.org/x/tools/gopls/internal/test/marker","Test":"Test/codeaction/removeparam_resolve.txt"}
[…]
{"Time":"2025-06-20T12:46:00.037725-04:00","Action":"cont","Package":"golang.org/x/tools/gopls/internal/test/marker","Test":"Test/codeaction/removeparam_resolve.txt"}
{"Time":"2025-06-20T12:46:00.037727-04:00","Action":"output","Package":"golang.org/x/tools/gopls/internal/test/marker","Test":"Test/codeaction/removeparam_resolve.txt","Output":"=== CONT  Test/codeaction/removeparam_resolve.txt\n"}
{"Time":"2025-06-20T12:46:00.168447-04:00","Action":"output","Package":"golang.org/x/tools/gopls/internal/test/marker","Test":"Test/codeaction/removeparam_resolve.txt","Output":"panic: test timed out after 20s\n"}
{"Time":"2025-06-20T12:46:00.168471-04:00","Action":"output","Package":"golang.org/x/tools/gopls/internal/test/marker","Test":"Test/codeaction/removeparam_resolve.txt","Output":"\trunning tests:\n"}
{"Time":"2025-06-20T12:46:00.168473-04:00","Action":"output","Package":"golang.org/x/tools/gopls/internal/test/marker","Test":"Test/codeaction/removeparam_resolve.txt","Output":"\t\tTest/codeaction/removeparam_resolve.txt (0s)\n"}
[…]
{"Time":"2025-06-20T12:46:00.248881-04:00","Action":"output","Package":"golang.org/x/tools/gopls/internal/test/marker","Output":"FAIL\tgolang.org/x/tools/gopls/internal/test/marker\t20.629s\n"}
{"Time":"2025-06-20T12:46:00.248887-04:00","Action":"fail","Package":"golang.org/x/tools/gopls/internal/test/marker","Elapsed":20.629}
$ echo $?                                  
1

Notably, the "test timed out after ... running tests: ..." message was attached to the output of a single subtest that was in the process of running, "Test/codeaction/removeparam_resolve.txt", and not anywhere else. In this particular instance, that test hadn't finished running and has no corresponding "fail" event. As things stand, when looking at a visualization of structured test output, it can be easy to miss this information, which seems to be a regression compared to the normal non-structured test output where it's quite prominent.

Perhaps there's room for improvement here. Maybe the output would be better to attach at the package-level instead of the test-level, since it refers to multiple package tests that were running, if that's viable? Maybe something else can be done?

CC @golang/command-line, @adonovan, @cherrymui.