[flang][OpenMP] Added test case for OpenMP 5.0 specification based semantic...

[flang][OpenMP] Added test case for OpenMP 5.0 specification based semantic checks for parallel sections construct

Parallel sections directive borrows the semantic checks from both sections directive and parallel directive. Semantic checks for both are merged in main branch; this test case is added to make sure correct semantic checks upon merging the two.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D111438