build: CMake: made it easier to run tests in parallel

build: CMake: made it easier to run tests in parallel

As of now `make -jN check` executes test suite sequentially
(although it builds test executables in parallel). This takes
a bit too long, so it would be nice to run tests in parallel.
`ctest -jN` or `make test ARGS=-jN` runs tests concurrently,
however it does not build test executables, instead it fails
if any test binary is missing.

Also `test` target is a bit special and it's impossible to add
dependencies to it, see
https://gitlab.kitware.com/cmake/cmake/issues/8438

To work around the problem define `test_suite` target which
builds the tests suite without running it, so one can both
compile and run tests in parallel

make -j8 test_suite
make test ARGS=-j8