GPUUploadManager: add tests for Page::Writer::ScheduleTextureUpdates … #4432

Job	Run time
Linux -> Pre-checks	18s
Linux -> Clang-x64, Release	13m 10s
Linux -> GCC-x64, Release	10m 52s
Linux -> Clang-x64, Debug	13m 22s
Linux -> Clang-NO_GLSLANG-x64, Debug	6m 39s
Linux -> Clang-NO_GLSLANG-NO_HLSL-x64, Release	5m 44s
Linux -> GCC-x64, Debug	12m 35s
	1h 2m 40s

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