tests: add macro for generating function enter log message

The test-tpm2 test multiplexes a bunch of tests from a single
entrypoint test that creates the TPM2 connection. This means we only get
the nice log output which test we are looking for once for the
entrypoint.

Let's add a macro that allows it to nicely generate it for the inner
tests too and use it.

src/shared/tests.h

@@ -104,6 +104,9 @@ extern const TestFunc _weak_ __stop_SYSTEMD_TEST_TABLE[];
         REGISTER_TEST(test_##name, ##__VA_ARGS__); \
         static int test_##name(void)
 
+#define TEST_LOG_FUNC() \
+        log_info("/* %s() */", __func__)
+
 static inline int run_test_table(void) {
         _cleanup_strv_free_ char **tests = NULL;
         int r = EXIT_SUCCESS;