unit-tests: show location of checks outside of tests

Checks outside of tests are caught at runtime and reported like this:

 Assertion failed: (ctx.running), function test_assert, file test-lib.c, line 267.

The assert() call aborts the unit test and doesn't reveal the location
or even the type of the offending check, as test_assert() is called by
all of them.

Handle it like the opposite case, a test without any checks: Don't
abort, but report the location of the actual check, along with a message
explaining the situation.  The output for example above becomes:

 # BUG: check outside of test at t/helper/test-example-tap.c:75

... and the unit test program continues and indicates the error in its
exit code at the end.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-example-tap.c

@@ -72,6 +72,8 @@ static void t_empty(void)
 
 int cmd__example_tap(int argc, const char **argv)
 {
+	check(1);
+
 	test_res = TEST(check_res = check_int(1, ==, 1), "passing test");
 	TEST(t_res(1), "passing test and assertion return 1");
 	test_res = TEST(check_res = check_int(1, ==, 2), "failing test");