convert enhancements to pytest

[sneakers-the-rat]

Fix: #1718

Other stuff:

• Make pythongen, generator, rawloader, schemaloader accept Paths, at least nominally so type checkers stop complaining.
• same with snapshot
• add marks:
  • no_asserts: for keeping track of the tests that are just print statements, just snapshot comparisons, or otherwise don't make meaningful assertions - we'll want to consolidate these and make sure we have unit tests for all the behavior they test perhaps indirectly
  • strcmp: tests that primarily compare stringified values rather than programmatic values - these are fragile and indirect, break when the string representation of anything changes, and can't use __eq__ of the native object types. we'll also want to progressively clean these up.
• started making tests xfail rather than skip so we know when we actually fix them/remind ourselves that we intended to turn them back on eventually
• add xfailing tests for pythongen pattern validation
• Generator had a log_level param but didn't use it, so i made it use it.

one step closer to not having to dodge outputs!

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 80.34%. Comparing base (bdabda1) to head (d246a16).

Files	Patch %	Lines
linkml/utils/rawloader.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2052   +/-   ##
=======================================
  Coverage   80.34%   80.34%           
=======================================
  Files         107      107           
  Lines       11967    11978   +11     
  Branches     3418     3422    +4     
=======================================
+ Hits         9615     9624    +9     
- Misses       1783     1784    +1     
- Partials      569      570    +1     

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