Some RunCMake tests unset some environment variables that affect ctest's
behavior before running it. Any test using the RunCTest helper needs to
do this, so do it in the helper itself.
Issue: #24153
When running tests in a non-fresh build tree there may be files left
from previous test runs. In the case that a test removes
`CTestConfig.cmake.in`, we must remove any `CTestConfig.cmake` that
may have been left behind.
