Set privacy.resistFingerprinting.testing.setTZtoUTC as a defense-in-depth

See https://bugzilla.mozilla.org/show_bug.cgi?id=1709867#c21 - we're changing how to spoof the timezone to make way for fine-grained control of that aspect of RFP. On Nightly we by default have changed the behavior of RFP. Tor Browser should flip this pref to true to keep the old behavior until we are certain their are no leaks. This will be landing on Nightly soon, so eventually it will ride to Release and affect Android.