For policy-specific tests, use the version before the policy was
introduced. Otherwise, use 3.5 where possible.
Also, remove `cmake_minimum_required()` and `project()` calls from
individual cases where they are handled by `CMakeLists.txt`.
When `string(REGEX REPLACE)` or `string(REGEX MATCHALL)` loop
internally, they store their matches, but they do not clear the previous
match from an earlier iteration. This can leave the contents of
`CMAKE_MATCH_<N>` with bogus values for later matches in the string if
they have groups which earlier matched a non-empty string, but now match
an empty string.
Fixes#17079.
