test: drop TEST_DATA_DIR, fold into get_testdata_dir()

Drop the TEST_DATA_DIR macro as this was using alloca() within a
function call which is allegedly unsafe. So add a "suffix" argument to
get_testdata_dir() instead and call that directly.

src/test/test-path.c

@@ -262,7 +262,7 @@ int main(int argc, char *argv[]) {
         log_parse_environment();
         log_open();
 
-        assert_se(set_unit_path(TEST_DATA_DIR("/test-path/")) >= 0);
+        assert_se(set_unit_path(get_testdata_dir("/test-path")) >= 0);
         assert_se(runtime_dir = setup_fake_runtime_dir());
 
         for (test = tests; test && *test; test++) {