cmd/go: shorten longest 5 tests

cmd/go: shorten longest 5 tests

Running go test -parallel=1 -v, these are the top 10 tests:

PASS: TestScript/mod_invalid_version 40.14s
PASS: TestScript/build_cache_output 46.82s
PASS: TestScript/get_legacy 48.69s
PASS: TestTestCache 58.44s
PASS: TestScript/mod_get_direct 62.88s
PASS: TestScript/build_pgo_auto_multi 63.49s
PASS: TestScript/build_pgo_auto 70.69s
PASS: TestScript/gcflags_patterns 95.17s
PASS: TestScript/mod_list_compiled_concurrent 124.31s
PASS: TestScript/vet_flags 202.85s

Change the top 5 not to run builds at all, so they don't
have to use -a or clear the go build cache.

mod_get_direct should be replaced with a vcs-test test.
mod_invalid_version should be replaced with a vcs-test test.
get_legacy should be deleted eventually.

Change-Id: Id67c458b1a96c912d89cbece341372c2ef5ee082
Reviewed-on: https://go-review.googlesource.com/c/go/+/504536
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>