Add McStas unittest for ALLOW_BACKPROP macro. #517

Job	Run time
windows-latest.msmpi.python-3.12	9m 40s
macos-latest.mpich.python-3.12	5m 16s
ubuntu-latest.openmpi.python-3.12	3m 39s
ubuntu-latest.mpich.python-3.12	3m 21s
macos-latest.openmpi.python-3.12	5m 10s
	27m 6s

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