Deprecate PymatgenTest, migrate pymatgen tests to pytest from unittest

[DanielYang59]

Summary

• Migrate pymatgen tests to pytest, to close [Dev/Test] Migrate PymatgenTest from unittest.TestCase to pytest #4202
• conftest.py doesn't seem to be used at all
• Add tests for legacy PymatgenTest to avoid accidental breakage

Warning

• [NEED CONFIRM] Deprecate PymatgenTest with MatSciTest, set deadline at 2026-01-01

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setUp -> setup_method even when not inheriting from TestCase

Test classes that doesn't subclass TestCase and define setUp method would work in pytest, but it's not a native support, so I would still migrate them here:

One thing that might catch users by surprise is that plain setup and teardown methods are not pytest native, they are in fact part of the nose support.
Native pytest support uses setup_method and teardown_method (see Method and function level setup/teardown), so the above should be changed to:

[DanielYang59]

I might remove this test as it is hard-coded to check the content of a non-existent file:

pymatgen/tests/command_line/test_gulp_caller.py

Lines 139 to 142 in 31f1e1f

	@unittest.expectedFailure
	def test_library_line_explicit_path(self):
	gin = self.gio.library_line("/Users/mbkumar/Research/Defects/GulpExe/Libraries/catlow.lib")
	assert "lib" in gin

[DanielYang59]

I originally didn't plan to deprecate PymatgenTest in this PR, but it turns out we have to do this here, otherwise tests that subclassing PymatgenTest would still rely on the TestCase and pytest methods like setup_method/class would not be compatible, meaning we have to do another migration after PymatgenTest dropped TestCase inheritance.

So I guess the easiest way is not to drop inheriting TestCase by PymatgenTest, but replacing PymatgenTest with another one that doesn't subclassing TestCase.

I drafted a name here but free feel to comment if you have better ideas (batch rename would be super easy with an IDE so don't worry).