runtime: eliminate arbitrary timeout in TestStackGrowth

Instead, allow the test to run up until nearly the test's deadline,
whatever that may be, and then crash with a panic (instead of calling
t.Errorf) to get a useful goroutine dump.

With the arbitrary timeout removed, we can now also run this test in
short mode, reducing its impact on test latency.

Fixes #19381

Change-Id: Ie1fae321a2973fcb9b69a012103363f16214f529
Reviewed-on: https://go-review.googlesource.com/c/go/+/378034


Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>