test: Add simple tests for "notmuch search --output=<>"

This code simply wasn't being exercised by the test suite before, so
this will be useful.

Meanwhile, there's currently a bug in "notmuch search --output=tags"
in that it doesn't print a final newline. But the current test suite
isn't able to catch this bug since the $() construct of the shell
doesn't preserve the distinction of whether the final newline is
present or not.