runners/datetime: fix testRemoteTimeInfo

From etcetera:

Be consistent with POSIX TZ settings in the Zone names,
even though this is the opposite of what many people expect.
POSIX has positive signs west of Greenwich, but many people expect
positive signs east of Greenwich.  For example, TZ='Etc/GMT+4' uses
the abbreviation "-04" and corresponds to 4 hours behind UT
(i.e. west of Greenwich) even though many people would expect it to
mean 4 hours ahead of UT (i.e. east of Greenwich).