LibompMicroTests.cmake: Invoke 'test' instead of using '[ x -eq y ]'

I was getting this cmake error on Mac OS X:

  CMake Error: Error in cmake code at
  /tmp/openmp/runtime/cmake/LibompMicroTests.cmake:140:
  Parse error.  Function missing ending ")".  Instead found bad character with text "[".

Perhaps invoking 'test' is less confusing for cmake.

Differential Revision: http://reviews.llvm.org/D11493

llvm-svn: 243165