[ENH] Add metadata pytest fixtures to reduce code duplication in tests

[PranavBhatP]

Is your feature request related to a problem? Please describe.
The metadata property is being frequently accessed across multiple test functions in test_data_module.py.

Suggested fixes
Creating pytest fixtures for metadata would eliminate the repeated need for calling

metadata = data_module.time_series_metadata

and then individually accessing the metadata, rather we could unfold the metadata within this fixture and reuse it. This was just an observation and would be a good fix early on to ensure similar practice in future tests.

[PranavBhatP]

FYI @phoeenniixx. I'm trying to implement similar changes for the tslib d2 module tests. You can open a PR for this, if you wish, since these will be non-breaking changes.