test: Set fixed time zone

test: Set fixed time zone

When the test suite is run in a different time zone that where Carl
lives, some tests may fail depending on the time when the test suite is
run. For example, just now I get:

     Search for all messages ("*"):...			FAIL
    --- test-031.expected	2010-04-23 09:33:47.898634822 +0200
    +++ test-031.output	2010-04-23 09:33:47.898634822 +0200
    @@ -1,5 +1,5 @@
    -thread:XXX   2001-01-05 [1/1] Notmuch Test Suite; Test message #6 (inbox unread)
    -thread:XXX   2001-01-05 [1/1] Notmuch Test Suite; Test message #14 (inbox unread)
    +thread:XXX   2001-01-06 [1/1] Notmuch Test Suite; Test message #6 (inbox unread)
    +thread:XXX   2001-01-06 [1/1] Notmuch Test Suite; Test message #14 (inbox unread)
     thread:XXX   2000-01-01 [1/1] Notmuch Test Suite; body search (inbox unread)
     thread:XXX   2000-01-01 [1/1] searchbyfrom; search by from (inbox unread)
     thread:XXX   2000-01-01 [1/1] Notmuch Test Suite; search by to (inbox unread)

By setting a fixed time zone in the test script, these problems should
be eliminated.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>