[8xx] Testing improvements [skip ci]

[theopnv]

Purpose of this PR

Joint backports of

• Disable playmode_XR tests with Metal #1980: Disable playmode_XR tests with Metal
• Split build & Execution for standalone tests (smoke, linux) [skip ci] #1680: Split build & Execution for standalone tests (smoke, linux)

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Testing status

• [canceled - result is not relevant] playmode_xr test was deactivated in the all_universal pipeline: https://yamato.cds.internal.unity3d.com/jobs/902-Graphics/tree/8xx%252Fjoint-backports/.yamato%252Fall-universal.yml%2523All_Universal_fast-2020.1/3774676/job/pipeline
• smoke tests have a dedicated pipeline and the standalone job has a build step: https://yamato.cds.internal.unity3d.com/jobs/902-Graphics/tree/8xx%252Fjoint-backports/.yamato%252Fall-smoke_test.yml%2523All_Smoke_Test_fast-2020.1/3774696/job/pipeline
• HDRP linux standalone has a build step: https://yamato.cds.internal.unity3d.com/jobs/902-Graphics/tree/8xx%252Fjoint-backports/.yamato%252Fhdrp-linux-vulkan.yml%2523HDRP_Linux_Vulkan_Standalone_fast-2020.1/3774690/job/pipeline

[theopnv]

Moved to #217