Migrate Test Suite

• Adapt the entire pystencils test suite to the changes introduced with version 2.0.
• Clean up the tests and separate them into subfolders.
• Mark still-failing tests as xfail
• Reintroduce tests-and-coverage CI task to run the full test suite
• Temporarily reduce coverage threshold to 80%
• Remove tests relating only to the old code generation backend

Don't be too afraid of the number of changes; most of them are just file moves.

src/pystencils/bit_masks.py

+from .sympyextensions.bit_masks import flag_cond as _flag_cond
+
+from warnings import warn
+warn(
+    "Importing the `pystencils.bit_masks` module is deprecated. "
+    "Import `flag_cond` from `pystencils.sympyextensions` instead."
+)
+
+flag_cond = _flag_cond