WIP: Adding fVDB unit tests #2343

Job	Run time
linux-nanovdb:cxx:g++-Release	1m 37s
linux-nanovdb:cxx:g++-Debug	1m 33s
windows-nanovdb (x64-windows-static, Release, -A x64 -G \"Visual Studio 17 2022\" -DOPENVDB_CORE_...	16m 39s
linux-nanovdb:cxx:clang++-Release	1m 29s
windows-nanovdb (x64-windows-static, Debug, -A x64 -G \"Visual Studio 17 2022\" -DOPENVDB_CORE_SH...	17m 44s
linux-nanovdb:cxx:clang++-Debug	1m 27s
windows-nanovdb (x64-windows, Release, -A x64 -G \"Visual Studio 17 2022\" -DOPENVDB_CORE_STATIC=...	22m 34s
windows-nanovdb (x64-windows, Debug, -A x64 -G \"Visual Studio 17 2022\" -DOPENVDB_CORE_STATIC=OFF)	21m 39s
macos-nanovdb (macos-11, clang++, Release)	1s
macos-nanovdb (macos-11, clang++, Debug)	1s
nanovdb-lite	2m 50s
	1h 27m 34s

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