cmd/go: convert TestCoveragePattern to the script framework

cmd/go: convert TestCoveragePattern to the script framework

This test already runs in parallel, but still convert it to the
script framework so we can delete the testdata/src directory and
remove any ambiguity about which tests can run in parallel.

Updates #36320

Change-Id: I6470979bd8bad0631dc6ead0d4eb9c83878356e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/212815
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>