explicitly set timezone for tests that depend on it

explicitly set timezone for tests that depend on it

Some timefunc tests rely on the time zone to be CE[S]T and will
fail otherwise. Not a good thing for a correctness test to do, so
set the zone explicitly in those cases. Example:

    $ ./test/test_i2ncommon --run_test=TestTimeFunc/FormatFullTime
    Running 1 test case...

    *** No errors detected
    $ TZ=UTC ./test/test_i2ncommon --run_test=TestTimeFunc/FormatFullTime
    Running 1 test case...
    /home/philipp/src/i2ncommon/libi2ncommon-git/test/test_timefunc.cpp(480): error: in "TestTimeFunc/FormatFullTime": check "17.10.2011 11:33" == format_full_time(seconds) has failed [17.10.2011 11:33 != 17.10.2011 09:33]