Add owners for /.test.mk and /.github

Add code owners for CI-related script and
for github automation directory.

NO_CHANGELOG=no code changes
NO_TEST=no code changes
NO_DOC=no code changes

(cherry picked from commit 9234763a)

.github/CODEOWNERS

 # Documentation team is in charge of changelogs.
 # No changes should go w/o approval by doc team
 /changelogs/ @tarantool/doc
+/.github/    @tarantool/devx
 /test/       @tarantool/devx
+/.test.mk    @tarantool/devx
 /test-run/   @tarantool/devx