internal/trace: corruption in tests

#!watchflakes
default <- pkg == "internal/trace" && test == "TestTraceGOMAXPROCS/Default"

Issue created automatically to collect these failures.

Example (log):

=== RUN   TestTraceGOMAXPROCS/Default
    trace_test.go:614: stderr: SIGBUS: bus error
        PC=0x52590 m=4 sigcode=2 addr=0xc00001a1600002
        
        goroutine 0 gp=0xc0003461c0 m=4 mp=0xc000342008 [idle]:
        runtime.runqempty(...)
        	/home/swarming/.swarming/w/ir/x/w/goroot/src/runtime/proc.go:6672
        runtime.checkRunqsNoP({0xc00002c000?, 0x0?, 0x0?}, {0xc00000e12c?, 0x0?, 0x0?})
        	/home/swarming/.swarming/w/ir/x/w/goroot/src/runtime/proc.go:3741 +0x130 fp=0xc000359dd8 sp=0xc000359db8 pc=0x52590
        runtime.findRunnable()
...
        r20  0x0	r21  0x1
        r22  0xc0003461c0	r23  0xc000359d38
        r24  0xc000342008	r25  0x0
        r26  0xffffffff13ad7807	r27  0xffffffffecca5c93
        r28  0xbaf106b	r29  0xea398
        r30  0xffffffffffffffc0	r31  0x68216d32
        pc   0x52590	link 0x51194
        exit status 2
    trace_test.go:616: exit status 1
--- FAIL: TestTraceGOMAXPROCS/Default (0.49s)

— watchflakes