[OpenMP] Extend the lit test for uses_allocators in target region

This patch improves the LIT tests on the following :

1. The test on `uses_allocators` clause in the `target` region by
adding the respective CHECK lines. Allocator `omp_thread_mem_alloc`
is also added in the test.
2. The `defaultmap` clause wasn't being tested for the variable-
category `scalar` and the implicit-behavior `tofrom` with respect
to the OpenMP default version.

These improvements are inspired from SOLLVE tests.
SOLLVE repo: https://github.com/SOLLVE/sollve_vv

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D132855