internal/testenv: add MustHaveSource, rm HasSrc

internal/testenv: add MustHaveSource, rm HasSrc

All the users of HasSrc call t.Skip anyway, so let's move it to testenv.

Fix go/build to use MustHaveSource rather than MustHaveGoBuild where
appropriate.

Change-Id: I052bf96fd5a5780c1930da5b3a52b7a8dbebea46
Reviewed-on: https://go-review.googlesource.com/c/go/+/612057
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Tim King <taking@google.com>
Auto-Submit: Tim King <taking@google.com>