Replace ok() by like() or is() at various places, due to Test::More

In a previous commit, the test module was changed from Test to Test::More.
In Test::More, ok() checks only for any true value, the second argument is
the name of the test. For detailed checks, it has to be replaced
by like() or is(), where appropriate.
Merge remote-tracking branch 'refs/remotes/origin/test-replace-ok-by-like'