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.

libgomp/testsuite/libgomp.oacc-fortran/collapse-1.f90

@@ -14,6 +14,9 @@ program collapse1
         end do
       end do
     end do
+  !$acc end parallel
+
+  !$acc parallel
   !$acc loop collapse(2) reduction(.or.:l)
     do i = 1, 3
       do j = 4, 6

libgomp/testsuite/libgomp.oacc-fortran/collapse-2.f90

@@ -13,6 +13,9 @@ program collapse2
         do 164 k = 5, 7
           a(i, j, k) = i + j + k
 164      end do
+  !$acc end parallel
+
+  !$acc parallel
   !$acc loop collapse(2) reduction(.or.:l)
 firstdo: do i = 1, 3
       do j = 4, 6