runtime/coverage: skip more tests in short mode

runtime/coverage: skip more tests in short mode

Add more skips if short mode testing, since some of these tests
still seem to be timing out on smaller and more underpowered
builders.

Updates #56197.

Change-Id: I469d9fd3a6be5602243234562fa3fe6263968b56
Reviewed-on: https://go-review.googlesource.com/c/go/+/443376
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>