openacc: Fix race condition in Fortran loop collapse tests

openacc: Fix race condition in Fortran loop collapse tests

The gangs participating in a gang-partitioned loop are not all guaranteed
to complete before some given gang continues to execute beyond that loop.
This means that two existing test cases contain a race condition,
because a loop that may be gang-partitioned is followed immediately by
another loop.  The fix is to place the loops in separate parallel regions.

2020-09-08  Julian Brown  <julian@codesourcery.com>

libgomp/
	* testsuite/libgomp.oacc-fortran/collapse-1.f90: Fix race condition.
	* testsuite/libgomp.oacc-fortran/collapse-2.f90: Likewise.