test: Disentangle execution of notmuch from the examination of the results.

test: Disentangle execution of notmuch from the examination of the results.

The old execute_expecting function was doing far too much for its own
good.  One of the worst aspects of this was that it introduced
shell-quoting challengers where the caller could not easily control
the precise invocation of the command to be executed.

I personally couldn't find a way to test "notmuch search '*'" without
the shell expanding * against files in the current directory, or
having bogus quotation marks appearing in the search string,
(defeating the recognition of "*" as a special search term).

Hopefully this aspect of the test suite will be much easier to maintain now.