[CI][Test] The behavior of incremental test is not correct sometimes

Issue:
When the diff between the PR branch (source) and master (extension repo)/dev (cli repo) branch (target) contains not only the PR changes, the incremental test will test the current PR changes as well other changes and it's not a correct behavior as incremental test should only test the changes in the current PR. This will cause some PRs fail to pass the CI.

Example: https://github.com/Azure/azure-cli-extensions/pull/1892

Potential solution:
CI rebases the PR branch on latest master/dev branch, then diff between the PR branch and master/dev branch, then test based on the diff.
Currently we leverage ADO diff functionality, if it doesn't support the solution above, we need use git commands ourselves.

Current workaround:
PR owner rebases the PR branch on latest master/dev branch manually.