GPUUploadManager: add tests for Page::Writer::ScheduleBufferUpdates i… #4429

Job	Run time
Linux -> Pre-checks	18s
Linux -> GCC-x64, Release	10m 43s
Linux -> Clang-NO_GLSLANG-x64, Debug	8m 24s
Linux -> Clang-x64, Debug	14m 2s
Linux -> Clang-NO_GLSLANG-NO_HLSL-x64, Release	6m 49s
Linux -> GCC-x64, Debug	12m 4s
Linux -> Clang-x64, Release	13m 33s
	1h 5m 53s

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