[libgomp, testsuite, nvptx] Fix dg-output test in vector-length-128-7.c

[libgomp, testsuite, nvptx] Fix dg-output test in vector-length-128-7.c

When running test-case libgomp.oacc-c-c++-common/vector-length-128-7.c on an
RTX A2000 (sm_86) with driver 510.60.02 I run into:
...
FAIL: libgomp.oacc-c/../libgomp.oacc-c-c++-common/vector-length-128-7.c \
  -DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none  -O0  \
  output pattern test
...

The failing check verifies the launch dimensions:
...
/* { dg-output "nvptx_exec: kernel main\\\$_omp_fn\\\$0: \
                launch gangs=1, workers=8, vectors=128" } */
...
which fails because (as we can see with GOMP_DEBUG=1) the actual num_workers
is 6:
...
  nvptx_exec: kernel main$_omp_fn$0: launch gangs=1, workers=6, vectors=128
...

This is due to the result of cuOccupancyMaxPotentialBlockSize (which suggests
'a launch configuration with reasonable occupancy') printed just before:
...
cuOccupancyMaxPotentialBlockSize: grid = 52, block = 768
...
[ Note: 6 * 128 == 768. ]

Fix this by updating the check to allow num_workers in the range 1 to 8.

Tested on x86_64 with nvptx accelerator.

libgomp/ChangeLog:

2022-04-01  Tom de Vries  <tdevries@suse.de>

	* testsuite/libgomp.oacc-c-c++-common/vector-length-128-7.c: Fix
	num_workers check.