MFC r326861-r326863:
Add some gmirror tests and fix indentation in existing tests.

MFC note: tests/sys/kern/coredump_phnum_test.sh changes omitted

MFC r322214:

Make test scripts under tests/... non-executable

Executable bits should be set at install time instead of in the repo.
Setting executable bits on files triggers false positives with Phabricator.

MFC r317713:
r317713 (by markj):

Add regression tests for r317712 and r306743.