Tests

Fix lots of clang-tidy issues in fv_ops #4172

Job	Run time
CUDA	12m 52s
Fedora	18m 26s
CMake, PETSc unreleased, ADIOS2	26m 7s
Default options, Ubuntu 20.04	17m 29s
Optimised, static	21m 34s
Debug, shared	25m 27s
Shared, release, Ubuntu 20.04	17m 50s
Shared, OpenMP, 3D metrics	14m 23s
CMake, new PETSc	18m 30s
	2h 52m 38s

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