Prevent CUDA context errors when testing on single-GPU

[pentschev]

Add a DASK_CUDA_TEST_SINGLE_GPU environment variable that allows
informing a single-GPU system is used for testing (such as gpuCI). This
then prevents throwing errors when attempting to mock create CUDA
context on devices that are specified via CUDA_VISIBLE_DEVICES but are
unavailable in the system.

[madsbk]

LGTM

[pentschev]

Thanks @madsbk for the lightning fast review! 😄

[codecov-commenter]

Codecov Report

Merging #737 (bb7c9d4) into branch-21.10 (8e6ab70) will increase coverage by 1.53%.
The diff coverage is 86.39%.

@@               Coverage Diff                @@
##           branch-21.10     #737      +/-   ##
================================================
+ Coverage         87.63%   89.16%   +1.53%     
================================================
  Files                15       15              
  Lines              1658     1911     +253     
================================================
+ Hits               1453     1704     +251     
- Misses              205      207       +2     

Impacted Files	Coverage Δ
dask_cuda/cuda_worker.py	77.64% <ø> (ø)
dask_cuda/get_device_memory_objects.py	90.00% <0.00%> (+21.94%)	⬆️
dask_cuda/utils.py	81.49% <60.52%> (-5.78%)	⬇️
dask_cuda/proxify_device_objects.py	95.45% <80.00%> (+6.56%)	⬆️
dask_cuda/initialize.py	89.79% <84.37%> (+0.90%)	⬆️
dask_cuda/proxify_host_file.py	93.51% <90.66%> (-5.88%)	⬇️
dask_cuda/proxy_object.py	91.34% <94.44%> (+1.70%)	⬆️
dask_cuda/device_host_file.py	71.66% <100.00%> (+1.50%)	⬆️
dask_cuda/explicit_comms/dataframe/shuffle.py	98.69% <100.00%> (+0.65%)	⬆️
dask_cuda/is_device_object.py	92.59% <100.00%> (+29.62%)	⬆️
... and 14 more

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[pentschev]

@gpucibot merge