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`.
Prior to commit 7c4c13ffef (math: Reject unexpected expression input
explicitly, 2018-05-18) we ignored unexpected characters in an
expression that otherwise can be parsed. In order to preserve
compatibility with projects that accidentally used this, convert the
error to a warning.
