Skip viscoacoustic tests with openacc

[rhodrin]

Owing to what looks like a compiler bug, the viscoacoustic tests will be skipped for the time being if openacc offloading to gpu.

See issue #1560.

[codecov]

Codecov Report

Merging #1559 (0cbbb46) into master (c1c513a) will decrease coverage by 8.76%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1559      +/-   ##
==========================================
- Coverage   85.86%   77.10%   -8.77%     
==========================================
  Files         210      192      -18     
  Lines       30949    29351    -1598     
  Branches     4158     3949     -209     
==========================================
- Hits        26574    22630    -3944     
- Misses       3893     6192    +2299     
- Partials      482      529      +47     

Impacted Files	Coverage Δ
conftest.py	52.03% <ø> (ø)
...les/seismic/viscoacoustic/viscoacoustic_example.py	48.83% <100.00%> (-21.17%)	⬇️
tests/test_mpi.py	0.90% <0.00%> (-98.10%)	⬇️
devito/mpi/routines.py	27.56% <0.00%> (-67.75%)	⬇️
devito/data/utils.py	34.93% <0.00%> (-57.65%)	⬇️
tests/test_data.py	40.69% <0.00%> (-56.93%)	⬇️
devito/data/data.py	49.01% <0.00%> (-45.43%)	⬇️
devito/mpi/distributed.py	53.40% <0.00%> (-38.00%)	⬇️
examples/seismic/tti/operators.py	62.50% <0.00%> (-33.34%)	⬇️
tests/test_gpu_openmp.py	66.29% <0.00%> (-33.15%)	⬇️
... and 77 more

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