Remove inv of singular matrix in tests #2149

Job	Run time
test (InterfaceI, 1)	20m 32s
test (InterfaceI, 1.6)	37m 36s
test (InterfaceII, 1)	26m 45s
test (InterfaceII, 1.6)	40m 51s
test (InterfaceIII, 1)	19m 30s
test (InterfaceIII, 1.6)	29m 39s
test (Integrators_I, 1)	20m 36s
test (Integrators_I, 1.6)	25m 5s
test (AlgConvergence_I, 1)	18m 42s
test (AlgConvergence_I, 1.6)	31m 29s
test (AlgConvergence_II, 1)	25m 10s
test (AlgConvergence_II, 1.6)	31m 15s
test (AlgConvergence_III, 1)	27m 44s
test (AlgConvergence_III, 1.6)	49m 44s
test (Downstream, 1)	28m 30s
test (Downstream, 1.6)	18m 39s
test (ODEInterfaceRegression, 1)	22m 24s
test (ODEInterfaceRegression, 1.6)	16m 49s
test (Multithreading, 1)	19m 4s
test (Multithreading, 1.6)	12m 12s
	8h 42m 16s

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