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`.
Represent the input file path internally in canonical form. Otherwise
multiple `configure_file` calls that share the same input file but specify
it relative to different directories (e.g. via `../`) result in multiple
copies of the dependency on the rule to re-run CMake. This causes the
Ninja generator to emit duplicate phony build statements for these
dependencies, which generates an error with `-w dupbuild=err`, which
will be default in Ninja 1.9.
Also canonicalize the output path for consistency.
Add a test case.
Fixes: #18584
