[BUG] Some fast parquet tests fail if the time zone is not UTC

[revans2]

Describe the bug

FAILED ../../src/main/python/fastparquet_compatibility_test.py::test_reading_file_written_by_spark_cpu[Timestamp(not_null)0][DATAGEN_SEED=0, INJECT_OOM] - AssertionError: GPU and CPU int values are different at [0, 'a']
FAILED ../../src/main/python/fastparquet_compatibility_test.py::test_reading_file_written_with_gpu[Timestamp(not_null)0][DATAGEN_SEED=0, INJECT_OOM] - AssertionError: GPU and CPU int values are different at [0, 'a']

SPARK_HOME=~/src/spark_3.4.2/ TEST_PARALLEL=0 DATAGEN_SEED=0 ./run_pyspark_from_build.sh -k 'test_reading_file_written_with_gpu or test_reading_file_written_by_spark_cpu'

Note that I am running on a computer with the timezone configured to 'America/Chicago' , but TZ itself is not set to anything.

$echo $TZ

If I set to TZ='UTC' or TZ='America/Chicago' then it works just fine, so I think this is a bug in how our test code detects time zones. It is probably really minor.