test_detect_match_nonexistent_template: Make it check the right behaviour

`detect_match` should (and does) throw if the template name its given is
wrong, but the test looked like it just returned a non-matching
`MatchResult`.

This commit fixes this test to actually test the expected behaviour.
API: Allow treating `MatchResult` as a boolean

This is part of an effort to make the stbt match API more composible.
For instance with the addition of a `match()` method you will be able
to write something like:

    if match('template.png') or match('template2.png'):
        do_stuff()

The tests are updated to reflect the recommended way of expressing testing
for matches.