cmd/go: rename TestModLegacy* to TestLegacyMod*
cmd/go: rename TestModLegacy* to TestLegacyMod*

This preserves the property that -run=^TestMod selects exactly those in mod_test.go.

Change-Id: Ib186b4f56d598669f08440befe6d2baedc947298
Reviewed-on: https://go-review.googlesource.com/123360
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/vgo,golang/vgo