Fixes NVIDIA#9776.

The tests in `fastparquet_compatibility_test.py` check for compatibility between
Apache Spark, the Spark RAPIDS plugin, and fastparquet. In particular:
1. `test_reading_file_written_by_spark_cpu` checks if timestamp columns written
    with Apache Spark are read similarly with fastparquet and the plugin.
2. `test_reading_file_written_with_gpu` checks if timestamps written with
   the plugin are read the same on Apache Spark and fastparquet.

If the timezone is not set to "UTC", and the system timezone isn't "UTC" either,
the plugin falls back to CPU for read/write of Parquet timestamp columns. This would
cause the above tests not to run: the plugin can neither read nor write timestamps
on GPU.

Further, fastparquet seems to interpret timestamps written from Spark as being
in "UTC", regardless of the timezone settings. So on non-UTC timezones,
Apache Spark and fastparquet get different results for the same input.

For the two reasons above, it is best to only run the three-way timestamp comparison
tests in setups with "UTC" timezone.

This commit skips the timestamp tests described above, when a non-UTC timezone is
detected.

Signed-off-by: MithunR <[email protected]>