fix: make enzyme testing opt-in for now #3622

Job	Run time
test (1.10, ubuntu-latest, core_layers)	16m 18s
downgrade	1h 7m 53s
test (1.10, ubuntu-latest, normalize_layers)	13m 23s
test (1.10, ubuntu-latest, recurrent_layers)	19m 41s
test (1.10, ubuntu-latest, autodiff)	23m 21s
test (1.10, ubuntu-latest, misc)	28m 11s
test (1.10, ubuntu-latest, reactant)	10m 24s
test (1, ubuntu-latest, core_layers)	19m 27s
test (1, ubuntu-latest, normalize_layers)	16m 30s
test (1, ubuntu-latest, recurrent_layers)	22m 12s
test (1, ubuntu-latest, autodiff)	24m 4s
test (1, ubuntu-latest, misc)	34m 7s
test (1, ubuntu-latest, reactant)	15m 5s
test (1.10, macos-latest, all)	3h 55m 53s
test (1.10, windows-latest, all)	1h 18m 0s
	10h 24m 29s

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