Fix test for mixed-ness in Function.sub, and fix Cofunction.sub to ma… #6524

Job	Run time
Build Firedrake (real, default)	1h 3m 39s
Build Firedrake (complex, --complex, complex)	57m 7s
Set the Docker release tag	1s
Build Docker containers / docker_env / Build the firedrake-env container	33m 9s
Build Docker containers / docker_vanilla / Build the firedrake-vanilla container	16m 43s
Build Docker containers / docker_complex / Build the firedrake-complex container	17m 58s
Build Docker containers / docker_docdeps / Build the firedrake-docdeps container	17m 46s
Build Docker containers / docker_firedrake / Build the firedrake container	10m 44s
Build Docker containers / docker_jupyter / Build the firedrake-jupyter container	7m 0s
	3h 44m 7s

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