Make timestamp overrides optional in tests and add faketime test

[kounelisagis]

Currently in TileDB-Py we have a number of tests which override the timestamp for writes in order to avoid test failures when writes happen within the same millisecond, e.g.:

TileDB-Py/tiledb/tests/cc/test_cc.py

Line 124 in ff21497

arr.set_open_timestamp_start(1)

TileDB-Py/tiledb/tests/test_fragments.py

Line 100 in ff21497

with tiledb.SparseArray(uri, mode="w", timestamp=timestamp) as T:

In libtiledb 2.22.0 a sub-millisecond temporal disambiguation of random labels from a single-process is implemented (serialized ordering for writes).

To test this from Python, tests that use timestamp ordering overrides are modified. They now run both with and without overriding and they should succeed in both cases. This solution is temporary; in the future, code that uses ordering override will be removed, ensuring a robust solution.

Additionally, to directly check that the new feature is working as expected, libfaketime is utilized. More specifically the faketime wrapper comes in handy in our case to decrease the complexity and have a unified solution for both Linux and macOS.
By using faketime and setting the time multiplier to zero (faketime -f '+x0' python time_test.py), we are able to "freeze" time for the whole duration of the test which runs in a subprocess.
Installation of libfaketime means a new CI step. A new test file, test_timestamp_overrides is added to run write tests under faketime and check that the expected outcome holds.

Checking if libfaketime exists, means a system call to skip test.

[bekadavis9]

LGTM!

[ihnorton]

A few suggested changes, but overall great work on the test_timestamp_overrides. When I apply the changes here, copy just that file out

• run w/ pytest under tiledb-py<0.28, it fails (as expected)
• run with tiledb-py==0.28, it succeeds. Excellent!