doc: update "Testing and CI" in Collaborator Guide
Update "Testing and CI" in the Collaborator Guide. Remove redundant material. Shorten sentences. Remove incorrect material. (Specifically, we don't require test cases to be included in all pull requests that modify exectuable code. For example, if code is refactored, then passing existing tests is sufficient.)
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines