Tests: Verify policies are introduced only in "official" versions

Add the CMake.PolicyCheck test.

This test uses "git grep" to look for policies added in a "dated"
version of CMake. It will fail if a policy is added as of,
for example, CMake 2.8.11.20130828.

The intent is to prevent such constructs from making it into an
"official" release. Three instances actually appeared in the first
attempted release candidate for 2.8.12.

This test may sometimes yield false positives. After all, it's just
using a regular expression to detect this condition, and something
in a comment could possibly match it. As of right now, that's not
true, but it's easy to imagine such a comment being added.

The new test may also not catch all future problems of this sort.
However, it will catch problems of this sort for all code that follows
the present layout style in Source/cmPolicies.cxx.

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