cmake: allow running the tests in parallel like in the Makefile

[isaac-io]

The Makefile supports running tests in parallel using GNU Parallel. CMake runs the tests through ctest, and is also supposed to run on Windows, so GNU Parallel can't be used.

However, CTest natively supports running tests in parallel with the appropriate environment variable set (CTEST_PARALLEL_LEVEL). The problem is that users don't usually have it set, and they might also be used to the J environment variable as that's what the Makefile uses.

Add a CMake script in order to be able to set the CTEST_PARALLEL_LEVEL (if not set by the user) according to the J environment variable if it was set, or fall back to the CPU core count, as the Makefile does.

While at it, allow speeding up the tests on Linux by setting TEST_TMPDIR to /dev/shm (if exists) according to the Makefile logic (only when the sticky bit is set), otherwise falling back on /tmp if exists.

[isaac-io]

Tests in the CMake configuration are executed using ctest. The default ctest output format is two lines for each test, a start line and an end line.

How to test this:

1. Generate a CMake target configuration using your favourite generator (in the example below I'm using the Makefile generator, which is the default on Linux):

   rm -rf build && mkdir build && cd build
   cmake ..
   make -j$(nproc) check
   

2. When ctest starts running you're seeing $(nproc) start lines before you start seeing end lines. This is the indication that tests are running in parallel (you'll also see total runtime reduced, but you can only compare before and after).

[Yuval-Ariel]

QA passed: