[OpenMP] split execution of a long test into smaller parts.

This test is bottlenecked by heavy regex use (~0.6s per FileCHeck run) with the
content that can't be further fragmented. Instead, the test body is moved into a
common .inc file and test execution has been split into four roughly equal
parts. This reduces wall time for the test from 14s to ~3.5s.

Differential Revision: https://reviews.llvm.org/D85695