cmake: Display skipped tests as not run (#2567)

cmake: Display skipped tests as not run (#2567)

Currently, the run_regression.py script just returns 0 when we skip a
test due to a feature not supported by the current configuration.
Returning 0 marks those tests as passed. To make it more clear which
tests were skipped, this commit adds the `SKIP_RETURN_CODE` [0] property
to the regression tests and changes the regression script to return 77
for skipped tests. The feature is supported since at least CMake 3.0 [0].
For backwards compatibility with autotools, returning 77 for skipped
tests is only active when `--cmake` is passed to the run_regression.py
script.

[0] https://cmake.org/cmake/help/v3.0/prop_test/SKIP_RETURN_CODE.html