CI, STYLE Consolidate pygrep checks

[MarcoGorelli]

These checks can be consolidated using multi-line expressions and verbose regex

I think this should make it easier to extend them, so it should be straightforward to port over the remaining ones in code_checks.sh without having to keep increasing the number of hooks

It also reduces the total number of distinct checks (26 now, down from 31 on master), leading to less visual clutter every time we make a commit

The output will still be clear, as it'll just show the line in which an offending pattern was found. E.g. inserting import conftest into pandas/tests/io/formats/test_info.py results in:

$ pre-commit run unwanted-patterns-in-tests -a
Unwanted patterns in tests...............................................Failed
- hook id: unwanted-patterns-in-tests
- exit code: 1

pandas/tests/io/formats/test_info.py:2:import conftest

[jreback]

thanks @MarcoGorelli