testing: print names of running tests on test timeout

testing: print names of running tests on test timeout

Track the running tests and when they started,
so that we can report the running tests on a test timeout.

	% go test -timeout=5s
	panic: test timed out after 5s
	running tests:
		TestTCPSpuriousConnSetupCompletion (4s)

	... stack traces as usual ...

	% go test -run=Script -timeout=10s cmd/go
	vcs-test.golang.org rerouted to http://127.0.0.1:65168
	https://vcs-test.golang.org rerouted to https://127.0.0.1:65169
	go test proxy running at GOPROXY=http://127.0.0.1:65170/mod
	panic: test timed out after 10s
	running tests:
		TestScript (10s)
		TestScript/mod_get_patchcycle (0s)
		TestScript/mod_get_prefer_incompatible (0s)
		TestScript/mod_get_promote_implicit (0s)
		TestScript/mod_get_pseudo (0s)
		TestScript/mod_get_pseudo_other_branch (0s)
		TestScript/mod_get_pseudo_prefix (0s)
		TestScript/mod_get_test (0s)
		TestScript/mod_get_trailing_slash (0s)
		TestScript/mod_get_update_unrelated_sum (0s)
		TestScript/mod_gobuild_import (0s)
		TestScript/mod_gomodcache (0s)
		TestScript/mod_gonoproxy (0s)
		TestScript/mod_load_badchain (0s)
		TestScript/mod_overlay (0s)
		TestScript/test_fuzz_minimize (6s)
		TestScript/test_fuzz_minimize_dirty_cov (7s)

	... stack traces as usual ...

Change-Id: I3a6647c029097becc06664ebd76a2597c7ed7b8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/446176
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>