Remove buffer_manager / runtime dependency from some accessor tests

[fknorr]

This is a back-port from IDAG development.

There are some tests for accessors that used to logically depend on full runtime or at least a buffer_storage / the buffer manager before we switched to USM buffers, but don't anymore. One test that should have used the runtime directly had been re-written to use the buffer manager to work around a ComputeCpp compiler bug, which now is not a concern anymore either.

This PR pulls out pure accessor tests from buffer_manager_tests and makes them runtime-independent where possible in anticipation of buffer_manager's removal with the switch to IDAG scheduling.

[github-actions[bot]]

Check-perf-impact results: (4c65f1399a47e0eb1340f63004745b17)

:question: No new benchmark data submitted. :question:
Please re-run the microbenchmarks and include the results if your commit could potentially affect performance.

[coveralls]

Pull Request Test Coverage Report for Build 7774971382

• 0 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.09%) to 93.946%

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Totals
Change from base Build 7727056751:	0.09%
Covered Lines:	4995
Relevant Lines:	5187

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💛 - Coveralls