Make like() and unlike() in t/test.pl refuse non-qr// arguments

As I noted in v5.21.1-12-g826af13 we have subtle bugs in the test suite
because you can do e.g. like($@, '') now which'll be a passing test even
when we have an error, because $@ =~ // will be true.

I'm just changing t/test.pl to not accept non-Regexp arguments, and
fixing up a bunch of test failures that resulted from that. There might
still be more of these in tests that I'm just not running, I've also
changed some of these from $str =~ /foo/ to $str eq 'foo'
(i.e. s/like/is/) in cases where that appeared to work, but it might
break some systems.

Let's just find that out via the smokers.