Introduce the mock test base

[ArcturusZhang]

This is a reopen of PR #22445

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[ArcturusZhang]

Hey @m-nash I have resolved the comments, could you please take a look again?

BTW the CI failure is due to some glitch of downloading, not issues in the code

[pakrym]

Don't do this. This might break other tests that run in the same process after.

[pakrym]

Override the property instead.

[pakrym]

Same as above.