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`.
Add a string(CONCAT) command to simply concatenate input arguments
together. This will be useful for combining strings from different
quoting syntaxes. Add a RunCMake.string test covering these cases.
Brad King