Allow omitting part of the output of a process

Sometimes it's a pain to match the *exact* output of a process, so this adds a
placeholder where `[..]` indicates "any more input on this line". It can in
theory be used multiple times in a input string.
Ensure a message is printed on test failure

This previously relied on the test itself printing failure, but this is not
always the case for test which, for example, segfault. Instead this ensures that
*something* is always printed when a test fails, normally a short blurb about
what executable failed to run.