[BUG] Avoid using un-timezoned datetimes in the test suite

[vyasr]

Describe the bug
The pylibcudf test suite uses a number of datetime objects without specifying a timezone. That can result in tests that will be broken by having a non-UTC timezone setting because we rely on extracting the timezone-aware timestamp for constructing pylibcudf scalars.

Steps/Code to reproduce bug
On any system where /etc/localtime does not point to UTC, run python/pylibcudf/pylibcudf/tests/test_scalar.py::test_from_py or python/pylibcudf/pylibcudf/tests/test_scalar.py::test_from_py_with_dtype.

Expected behavior
We should specify a timezone in all datetime constructors so that we get consistent behavior regardless of the locale.