Switch to central definition of gather_global and unit test #1103

Job	Run time
Lint	16s
fenicsx-refs / Set FEniCSx component git refs	1s
Build and publish docs	4m 23s
Build and test	5m 9s
Build and test (linux-gnu-real32-32, ghcr.io/fenics/test-env:current-openmpi)	3m 56s
Build and test (linux-gnu-real64-32, ghcr.io/fenics/test-env:current-openmpi)	3m 58s
Build and test (linux-gnu-complex64-32, ghcr.io/fenics/test-env:current-openmpi)	3m 55s
Build and test (linux-gnu-complex128-32, ghcr.io/fenics/test-env:current-openmpi)	3m 59s
Build and test (linux-gnu-real64-64, ghcr.io/fenics/test-env:current-openmpi)	3m 58s
Build and test (linux-gnu-complex128-64, ghcr.io/fenics/test-env:current-openmpi)	4m 0s
Build and test (linux-gnu-real64-32, ghcr.io/fenics/test-env:current-mpich)	4m 0s
Build and test (linux-gnu-complex128-32, ghcr.io/fenics/test-env:current-mpich)	4m 0s
	41m 35s

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