[runtime] [test] Fix using %python path

Fix the newly-added tests to use %python substitution in order to use
the correct path to Python interpreter.  Otherwise, they fail on NetBSD
where there is no 'python', just 'pythonX.Y'.

Differential Revision: https://reviews.llvm.org/D56048

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@350001 91177308-0d34-0410-b5e6-96231b3b80d8