tests: Don’t use a potentially-existent timezone in a test for failure

When checking that the timezone code falls back to UTC if a zone is
unknown, don’t use ‘PST’, because that can actually exist on some
systems.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

GNOME/glib#1433
8 jobs for 1433-rename-timezones-in-tests in 19 minutes and 57 seconds (queued for 1 minute)
Status Job ID Name Coverage
  Build
passed #76082
cross-android_api21_arm64

00:01:35

passed #76083
cross-android_api28_arm64

00:01:33

passed #76084
cross-mingw64

00:02:09

passed #76081
fedora-x86_64

00:04:13

manual #76087
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #76085
win32
msys2-mingw32

00:08:21

passed #76086
win32
vs2017-x64

00:06:39

 
  Coverage
passed #76088
coverage

00:01:40

73.3%