cmd/go: remove tests using testdata/flag_test.go

Part of converting all tests to script framework to improve
test parallelism.

Updates #36320
Updates #17751

Change-Id: I6573185cf14f298c51f76265f18a75e4960ce791
Reviewed-on: https://go-review.googlesource.com/c/go/+/213220


Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>