tests: small improvement for Misc/verify-swift-feature-testing.test-sh

The `git grep` command fails if the repository has a branch named `test`.
Inserting a `--` in the git command line makes it clear that `test` is the directory and not the branch

test/Misc/verify-swift-feature-testing.test-sh

@@ -114,6 +114,7 @@ def find_matches(swift_src_root):
             # Be careful to not use RUN with a colon after it or Lit will pick
             # it up.
             "RUN[:].*-enable-(experimental|upcoming)-feature",
+            "--",
             "test",
             "validation-test",
         ],