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`.
Set the minimum required version of CMake high enough to avoid the
warning for CMAKE_LEGACY_CYGWIN_WIN32. The warning appears on stderr
and breaks the expected output matching.
When called with a non-existent LHS target name the user may be trying
to add file-level dependencies. Clarify the error message to explain
the difference between target-level and file-level dependencies. Point
the reader at the commands and options needed for the latter.
Brad King