[flang] Added test case for semantic check of OpenMP Master construct

Original-commit: flang-compiler/f18@22f3088c76
Reviewed-on: https://github.com/flang-compiler/f18/pull/808
Tree-same-pre-rewrite: false

flang/test/semantics/omp-clause-validity01.f90

@@ -377,6 +377,15 @@
     a = 3.14
   enddo
 
+! 2.13.1 master
+
+  !$omp parallel
+  !ERROR: NUM_THREADS clause is not allowed on the MASTER directive
+  !$omp master num_threads(4)
+  a=3.14
+  !$omp end master
+  !$omp end parallel
+
 ! Standalone Directives (basic)
 
   !$omp taskyield