CMake: Allow to define multiple threads= variant tests

CMake: Allow to define multiple threads= variant tests

This change allow to define multiple test flavors with threads=
similarly to how we handle mpirun=. For example,

  category/test.cc
  category/test.threads=2.output
  category/test.threads=4.output

will now create one top level target to compile test.cc and define two
tests "category/test.threads=2.(release|debug)" and
"category/test.threads=4.(release|debug)" that execute the test program
with a threadpool size set to 2 and 4, respectively.