This allows to generate a lot of simple files directly in the test tree instead of having them all checked in.
Provide a CMake-language binding to the md5sum function previously available only by "cmake -E md5sum".
The CMake.File test runs several scripts through "cmake -P" and checks the output and result against known good values. This commit factors out the checking code into a separate CMakeCheckTest module. The module may be used by new tests.