Expand test suite, including example output from subsetter

[danielfromearth]

The stitchee test suite has missed a few edge cases, so it should be expanded:

• include l2ss-py output as test data for concatenation tests
  • spatial
  • temporal
  • and variable-subset results
• create a download script (or embed as a test fixture), using harmony-py, so that the test data are re-generated when the subsetter is updated. (Can tests that rely on harmony-py be run in GitHub Actinos workflow? They would need credentials).
• ensure tests exist for CLD04, HCHO, NO2, O3TOT level-2 TEMPO datasets
• And add reasonably sized example data files for automated tests doing GitHub actions workflow

[danielfromearth]

Related to #11 .