[release-branch.go1.15] cmd/go/internal/test: keep looking for go command...

[release-branch.go1.15] cmd/go/internal/test: keep looking for go command flags after ambiguous test flag

Updates #40763
Fixes #40802

Change-Id: I275970d1f8561414571a5b93e368d68fa052c60f
Reviewed-on: https://go-review.googlesource.com/c/go/+/248618


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
(cherry picked from commit 797124f5)
Reviewed-on: https://go-review.googlesource.com/c/go/+/248726