cmd/go: migrate tests from `vcs-test.golang.org` to a test-local server

[jasonkeene]

There are a few tests that rely on vcs-test.golang.org. Do we need to have these tests talk to a vcs server on the internet or can the tests just start this server themselves? If the server can be ran locally I would think that would be better than having them rely on the availability of vcs-test.golang.org.

Follow up from: #27127 (comment)

cc/ @FiloSottile

[andybons]

@bcmills @rsc

[gopherbot]

Change https://go.dev/cl/418103 mentions this issue: cmd/go: convert TestAccidentalGitCheckout to a script test

[gopherbot]

Change https://go.dev/cl/417658 mentions this issue: cmd/go: remove testdata/modlegacy directory

[gopherbot]

Change https://go.dev/cl/419874 mentions this issue: cmd/go: avoid passing testing.T to isCaseSensitive and goVersion