[libgomp, testsuite] Scale down some OpenACC test-cases

[libgomp, testsuite] Scale down some OpenACC test-cases

When a display manager is running on an nvidia card, all CUDA kernel launches
get a 5 seconds watchdog timer.

Consequently, when running the libgomp testsuite with nvptx accelerator and
GOMP_NVPTX_JIT=-O0 we run into a few FAILs like this:
...
libgomp: cuStreamSynchronize error: the launch timed out and was terminated
FAIL: libgomp.oacc-c/../libgomp.oacc-c-c++-common/parallel-dims.c \
  -DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none -O0 \
  execution test
...

Fix this by scaling down the failing test-cases by default, and reverting to
the original behaviour for GCC_TEST_RUN_EXPENSIVE=1.

Tested on x86_64-linux with nvptx accelerator.

libgomp/ChangeLog:

2022-03-25  Tom de Vries  <tdevries@suse.de>

	PR libgomp/105042
	* testsuite/libgomp.oacc-c-c++-common/parallel-dims.c: Reduce
	execution time.
	* testsuite/libgomp.oacc-c-c++-common/vred2d-128.c: Same.
	* testsuite/libgomp.oacc-fortran/parallel-dims.f90: Same.