[openmp] Run tests on both runtimes, independent of the default

[JonChesterfield]

Summary

Minor fix to the lit.cfg. Currently, nvptx runs the tests twice on the new runtime.
Soon, amdgpu will run them on the new runtime as well as the old.

[jdoerfert]

seems reasonable

openmp/libomptarget/test/lit.cfg

else: # Unices
     if config.cuda_libdir:
         config.test_flags += " -Wl,-rpath," + config.cuda_libdir
     if config.libomptarget_current_target.startswith('amdgcn'):
         config.test_flags += " --libomptarget-amdgcn-bc-path=" + config.library_dir
     if config.libomptarget_current_target.startswith('nvptx'):
         config.test_flags += " --libomptarget-nvptx-bc-path=" + config.library_dir
     if config.libomptarget_current_target.endswith('-newRTL'):
         config.test_flags += " -fopenmp-target-new-runtime"
+    else:
+        config.test_flags += " -fno-openmp-target-new-runtime"
 
 # substitutions
 # - for targets that exist in the system create the actual command.
 # - for valid targets that do not exist in the system, return false, so that the
 #   same test can be used for different targets.
 
 # Scan all the valid targets.
 for libomptarget_target in config.libomptarget_all_targets: