Bad timezone test

[jamesjer]

My timezone is currently Mountain Standard Time (UTC - 7). Running the test suite fails on a timezone test:

ASSERT gap g6 Local = gap g6 UTC + gap UTC Local
File "tests/test_timezone.ml", line 37, character 2:
FAIL gap g6 Local = gap g6 UTC + gap UTC Local

   Expected: `11'
   Received: `-13'

(gap g6 Local) is -13, which make_in_bounds converts to 11. But (gap g6 UTC) is -6 and (gap UTC Local) is -7, so their sum is indeed -13. The test should do the equivalent of calling make_in_bounds on the result of the addition.