CI: use multiple threads to run CMake tests

[pwrobeldev]

The step, which is responsible for running CMake tests on CI is
extended to use multiple threads. This speeds up the execution.

It is achieved via 'CTEST_PARALLEL_LEVEL' environment variable.

[pwrobeldev]

The historical data from executions without CTEST_PARALLEL_LEVEL shows that CMake tests took around 270 seconds.

Some durations of running CMake tests -- three last executions of that job:

• Total Test time (real) = 270.18 sec
• Total Test time (real) = 272.38 sec
• Total Test time (real) = 273.42 sec

The usage of CTEST_PARALLEL_LEVEL=16 slowed-down the execution on CI (even though locally it helped):

• Total Test time (real) = 399.27 sec
• Total Test time (real) = 378.48 sec

The usage of 8 threads yields similar results to the initial durations:

• Total Test time (real) = 290.36 sec
• Total Test time (real) = 297.10 sec

The usage of 6 threads:

• Total Test time (real) = 261.29 sec
• Total Test time (real) = 270.71 sec

The usage of 4 threads:

• Total Test time (real) = 244.52 sec
• Total Test time (real) = 242.22 sec

The usage of 3 threads:

• Total Test time (real) = 244.15 sec
• Total Test time (real) = 236.41 sec