Implement a mechanism to discard tests based on cmake variables

Allow to specify the discardal of tests based on CMake variables from the meta ini file. Consider the following example:

[__static]
GRID = Yaspgrid<2>, UGGrid<2> | expand grid

Any test based on UG should be discarded if HAVE_UG is not true. This is not possible in the cc file anymore, as it is written generically.

This file introduces a solution approach quite similar to the 'exclude' command for constraints. The corresponding command is called 'cmake_guard'. Whenever the given condition evaluates to false in CMake, the test is discarded (aka not added to the testing suite). Note that you may have several such command lines: The test will be discarded if any of them evaluates to false.

To above example add the following line:

1, HAVE_UG | expand grid | cmake_guard

This fixes #94 (closed)