First tests for JSON output and UTF-8 in mail body and subject

First tests for JSON output and UTF-8 in mail body and subject

The test suite doesn't yet cover --format=json output nor UTF-8 in
subject or body.

This patch starts with test cases for 'search --format=json' and
'show --format=json'.

Furthermore, it has test cases for a search for a UTF-8 string in a mail
body for a UTF-8 string in a mail subject.

Finally, it has a test case for --format=json with UTF-8 messages,
demonstrating the fix in 1267697893-sup-4538@sam.mediasupervision.de.

Reviewed-by: Carl Worth <cworth@cworth.org>
Updated tests to current implementation of the test suite.
These tests demonstrate a bug in the current implementation
of "notmuch show --format=json", (timestamp output is changed
depending on current timezone).