Bug 9486: t/Letters.t loads modules in wrong order

Because the unit test t/Letters.t loads C4::Letters before C4::Context,
and C4::Letters is not even vaguely thread-safe, the test tends to fail.
Usually. Moving the dbh mocking to before the use_ok('C4::Letters') test
fixes the problem.

To test:
1) Before applying patch, run `prove t/Letters.t` a few times. Note that
   it fails most of the time, if not all the time.
2) Apply patch.
3) Repeat step (1), noting that now it passes every time.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Works as advertised.
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Merge branch 'bug_9486' into 3.12-master