Add support for running tests in parallel to the cmake build system.

Add support for running tests in parallel to the cmake build system.

Pass appropriate -jN option to ctest (via ARGS variable in the Makefile) to
enable support for running tests in parallel. Similarly to makefile build
system, ctest -j1 mode is enforced even when parallel mode in debhelper is not
explicitly enabled.

Unlike make, CTest does not have "unlimited parallel" setting (-j implies -j1).
So in order to simulate unlimited parallel, allow to fork a huge number of
threads instead.