Fix assert_str_content_equal, add tests for testing utils

[DanielYang59]

Summary

• Fix assert_str_content_equal not raising AssertionError
• Add unit tests for util.testing as it's intended for public usage (and quite some external code use PymatgenTest)
• More human-readable error messages for assert methods in PymatgenTest
• [NEED CONFIRM] Simplify single-file module testing/__init__.py to testing.py

---

Warning

[NEED CONFIRM] Simplify single-file module testing/__init__.py to testing.py (Is this intended for some purpose?), I don't think this would be breaking (though the definition of MODULE_DIR would change from util/testing to util, but no code import this AFAIK as the util/testing has nothing else, I believe it's intended for STRUCTURES_DIR and I have updated this)

---

Fix assert_str_content_equal not raising AssertionError

Current implementation would not raise AssertionError when two strings are not equal (it only returns a bool):

pymatgen/src/pymatgen/util/testing/__init__.py

Lines 66 to 69 in 31f1e1f

	def assert_str_content_equal(actual, expected):
	"""Test if two strings are equal, ignoring things like trailing spaces, etc."""
	strip_whitespace = {ord(c): None for c in string.whitespace}
	return actual.translate(strip_whitespace) == expected.translate(strip_whitespace)

And current usage of it take the pattern of self.assert_str_content_equal without assert.

Several issues found:

• test_str and test_mul for TestStructureGraph of analysis.graphs are failing #4206
• test_symmetry_ops for TestMagneticSpaceGroup of symmetry.maggroups is failing #4207

[DanielYang59]

Warning

[NEED CONFIRM] make MODULE_DIR private, as the util.testing directory contains nothing else other than__init__.py, I guess it's used to define STRUCTURES_DIR only:

pymatgen/src/pymatgen/util/testing/__init__.py

Lines 27 to 28 in 31f1e1f

	MODULE_DIR = Path(__file__).absolute().parent
	STRUCTURES_DIR = MODULE_DIR / ".." / "structures"

[DanielYang59]

More readable error message

Before: <class 'util.test_testing.TestAssertMSONable.test_not_round_trip.<locals>.NotAKpoints'> != <class 'pymatgen.io.vasp.inputs.Kpoints'>

Now: The reconstructed NotAKpoints object is not a subclass of Kpoints