For portability, absl_time_test builds a small, incomplete tzdata
database into the test binary and uses that instead of the system tzdata
database. (absl_time_test needs to run on some platforms that lack a
system tzdata database.) However, this causes issues if TZ is set to
something that isn’t in the test database. To address them, fall back to
America/Los_Angeles if TZ is set to an unknown value during testing.

Bug: https://bugs.debian.org/1012194
PiperOrigin-RevId: 453257912
Change-Id: I293d0f96876b31c32a2847468a3377bb49f3aa15