cmd/go/internal/modfetch/codehost: initialize localGitURL lazily and clean up skips
Previously localGitURL was initialized in TestMain, which creates
needless work if the test flags do not result in running a test that
requires localGitURL.
We had also been skipping a bunch of tests that used
vcs-test.golang.org in order to avoid network traffic, but now that
that content is served through an in-process vcweb server that is no
longer necessary. (All of the 'git' tests together take less than a
second to run.)
The 'hg' tests are much slower, so we do still skip those in short
mode.