I asked for feedback on the approach of implementing tests in .el[1] and
got only positive feedback (well, only one bit of feedback at all).

Here are some simpler tests implemented using this approach, trying to
address the concerns about seeing what failed. The test output is now
something like:

 PASS   notmuch-show-clean-address 1
 PASS   notmuch-show-clean-address 2
 FAIL   notmuch-show-clean-address 3
        Expect: "ДБ <db-uknot@stop.me.uk>"
        Output: "db-uknot@stop.me.uk"
        Expect: "foo (at home) <foo@bar.com>"
        Output: "foo <foo@bar.com>"
        Expect: "foo [at home] <foo@bar.com>"
        Output: "foo at home <foo@bar.com>"
        Expect: "Foo Bar"
        Output: nil

Obviously the first two tests passed and the third failed.

Footnotes: 
[1]  id:"cunmxabqs6k.fsf@hotblack-desiato.hh.sledj.net"