cmd/go: remove arbitrary sleep in TestScript/test_chatty_parallel_success_sleepy

cmd/go: remove arbitrary sleep in TestScript/test_chatty_parallel_success_sleepy

(Also rename it to no longer describe itself in terms of sleeping.)

This test previously relied on the scheduler to wake up a goroutine to
write the "--- PASS: TestFast" line within 100ms of TestFast actually
finishing. On some platforms, even that long a delay is apparently too
short.

Instead, we now use a deterministic "=== RUN" line instead of a
timing-dependent "--- PASS" line to interrupt the output.

Fixes #51221

Change-Id: I3997640fb7577e29e3866a82d4d49a3a70a4b033
Reviewed-on: https://go-review.googlesource.com/c/go/+/386154
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>