test: Quote $output in calls to test_expect_equal

test: Quote $output in calls to test_expect_equal

Previously, many tests in emacs-subject-to-filename didn't quote the
$output argument to test_expect_equal.  As a result, if $output was
empty, test_expect_equal would be passed only one argument and would
abort the entire test script.  By quoting the argument, we ensure
test_expect_equal will always receive two arguments.