requirements_test: Add pytest-xdist==2.2.1

[bcoles]

I've been using pytest-xdist locally for over a year to significantly speed up tests.

I forget if there was a reason we weren't already using this in requirements_test.txt for CI. Let's see if the tests pass.

[codecov-commenter]

Codecov Report

Merging #1178 (5f6f65d) into master (90947fc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1178   +/-   ##
=======================================
  Coverage   50.19%   50.19%           
=======================================
  Files         456      456           
  Lines       36794    36794           
=======================================
  Hits        18470    18470           
  Misses      18324    18324           

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[bcoles]

Ok tests pass. Let's try adding it to Travis.

[bcoles]

The tests completed successfully, but only about 16 seconds faster. Compare:

• https://travis-ci.org/github/smicallef/spiderfoot/jobs/774348380
• https://travis-ci.org/github/smicallef/spiderfoot/jobs/774346710

pytest correctly identified that two processor cores were available, so I'm not sure why there wasn't much speed up, unless Travis was already performing some multi-core magic. It is also possible that Travis CI infrastructure issues caused discrepancies.

I suspect that over time we'll see a generally lower average completion time.

I see no harm in merging these changes in.