testing: output hard to read when a test case is stalling

When the package contains a stalling case, the verbose is hard to read to figure out what has run and not finished finished yet. I often have to pipe the output to another program to match RUNs and PASSes (or FAILs) to filter what has not finished.

We might just consider printing the currently running test cases upon SIGINT.

go test -v
=== RUN   TestTrace
=== RUN   TestTraceWithWait
--- PASS: TestTraceWithWait (0.01s)
=== RUN   TestTraceFromHeader
=== RUN   TestTraceFromHeaderWithWait
--- PASS: TestTraceFromHeaderWithWait (2.00s)
=== RUN   TestNewSpan
--- PASS: TestNewSpan (0.01s)
=== RUN   TestNoTrace
--- PASS: TestNoTrace (0.04s)
=== RUN   TestNoTraceWithWait
--- PASS: TestNoTraceWithWait (0.14s)
=== RUN   TestNoTraceFromHeader
--- PASS: TestNoTraceFromHeader (0.04s)
=== RUN   TestNoTraceFromHeaderWithWait
--- PASS: TestNoTraceFromHeaderWithWait (0.04s)
=== RUN   TestSample
--- PASS: TestSample (0.00s)
=== RUN   TestSampling
=== RUN   TestBundling
=== RUN   TestWeights
--- PASS: TestWeights (0.02s)
=== RUN   TestPropagation
--- PASS: TestPropagation (0.00s)
=== RUN   TestNoTraceIncoming
--- PASS: TestNoTraceIncoming (0.00s)
--- PASS: TestSampling (1.98s)
--- PASS: TestTrace (2.00s)
--- PASS: TestTraceFromHeader (2.00s)
^Csignal: interrupt
FAIL	cloud.google.com/go/trace	189.182s