Specifier for a precise match using `-s`

[ethomson]

When using -s, it will match a prefix. For example -sfoo will match tests that start with foo. This is useful for matching categories like -sfoo::one, -sfoo::two, -sfoo::three, etc.

However, sometimes you want to be able to provide an exact match because you may have a test named foo::do_this_thing and foo::do_this_thing_and_another.

Now you have no way to run foo::do_this_thing since the prefix match will find both.

Now, this suggests a poor naming convention but test users may not be the app developer, and we should support them. Add a way to literally specify a thing, maybe using a $ at the end, eg -sfoo::do_this_thing$.