nitunit: testsuite+diff tries additional `.res` files

nitunit: testsuite+diff tries additional `.res` files

* "test_suite.sav/test_case.res" grouped by test-suites
  The current one.
  Useful if there is a lot of test-suites and test-cases in a directory
* "sav/test_case.res" just grouped in a common directory
  Useful if there is a lot of test-suites OR test-cases in a directory
* "test_case.res" raw in the directory.
  Useful is there is a few test-suites and test-cases in a directory.

All 3 are exclusive. If more than one exists, the test-case is failed.

Signed-off-by: Jean Privat <jean@pryen.org>