Allow tests to specify their internal use of multiple processes

[hpkfft]

Unit tests may be single threaded or may use threads internally to run on multiple processors.
I would like to share an idea for allowing the test() function to have a keyword argument processes which can be set to a positive integer. This integer indicates to meson that the test "consumes" multiple processes (i.e., uses multiple CPU cores).

Given, for example, MESON_NUM_PROCESSES=6, the idea would be to run combinations such as the following:

• One test with processes : 4 and two tests with processes : 1
• One test with processes : 4 and one test with processes : 2
• Three tests with processes : 2
• One test with processes : 2 and four tests with processes : 1
• etc.

[hpkfft]

I did not intend to imply that this is a novel idea. For example, in Cmake/Ctest:

set_tests_properties(my_test_name PROPERTIES PROCESSORS 4 PROCESSOR_AFFINITY True)

See also:
set_tests_properties
PROCESSORS
PROCESSOR_AFFINITY