TestStackGrowth is currently a parallel test. However, it depends on a
20 second timeout, which is already dubious in a parallel test, and
became really problematic on slow builders when runtime.GC switched to
triggering concurrent GC instead of STW GC. Before that change, the
test spent much of its time in STW GC, so it wasn't *really* parallel.
After that change, it was competing with all of the other parallel
tests and GC likely started taking ~4 times longer. On most builders
the whole test runs in well under a second, but on the slow builders
that was enough to push it over the 20 second timeout.

Fix this by making the test serial.

Updates #19381 (probably fixes it, but we'll have to wait and see).

Change-Id: I21af7cf543ab07f1ec1c930bfcb355b0df75672d
Reviewed-on: https://go-review.googlesource.com/48110
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Elias Naur <elias.naur@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>