cmd/go: convert some testcases to the script framework

This change converts TestFatalInBenchmarkCauseNonZeroExitStatus,
TestBenchmarkLabels and TestWildCardsDoNotLookInUselessDirectories
to the script framework so they can run in parallel. These tests
used the testdata directory so they didn't call tg.parallel, because
they couldn't be sure the testdata directory wasn't being
modified while they were being run.

The tests are converted to the script framework instead of being modified
because the script framework tests are easier to read and are self-contained.

Also remove the directory src/cmd/go/testdata/src/badc. The testcase
that used it, "TestDisallowedCSourceFiles" was deleted in
golang.org/cl/19613.

Updates #17751

Change-Id: I0b9b417ae1a9b148067a3e5f8531229f3414f104
Reviewed-on: https://go-review.googlesource.com/c/go/+/212620


Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>