Disable t/test-meta.sh in 'make test' unless TEST_META=1

Disable t/test-meta.sh in 'make test' unless TEST_META=1

In other words:

	make test                # doesn't run metadata tests
	TEST_META=1 make test    # does run metadata tests

The metadata tests still fail randomly on some people's computers, but we're
falling too far behind and it's time to make a release.  The metadata stuff
isn't used anywhere critical in bup yet, so it's okay to leave it in but not
test it for now.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>