Use pytest for `test_units` #1313

Job	Run time
Build MDANSE and run tests on macOS-latest	6m 55s
Build MDANSE and run tests on windows-latest	17m 0s
Build MDANSE and run tests on ubuntu-latest	14m 32s
Build MDANSE and run tests on ubuntu-latest	13m 46s
Build MDANSE and run tests on macOS-latest	9m 2s
Build MDANSE and run tests on macOS-latest	6m 39s
Build MDANSE and run tests on ubuntu-latest	13m 7s
Build MDANSE and run tests on windows-latest	17m 9s
Build MDANSE and run tests on ubuntu-latest	16m 26s
Build MDANSE and run tests on windows-latest	17m 33s
Build MDANSE and run tests on macOS-latest	7m 48s
Build MDANSE and run tests on windows-latest	17m 8s
Build MDANSE and run tests on macOS-latest	6m 41s
Build MDANSE and run tests on windows-latest	17m 47s
Build MDANSE and run tests on ubuntu-latest	14m 26s
	3h 15m 59s

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