Move and reuse TestHelper

[peterneyens]

I moved the "Async" DataSources into TestHelper, moved TestHelper to its own file and reused TestHelper in the tests of fetch-monix and fetch-twitter.

I minimized the imports of the test files in the meanwhile.

[codecov-io]

Codecov Report

Merging #132 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #132   +/-   ##
=======================================
  Coverage   78.81%   78.81%           
=======================================
  Files          15       15           
  Lines         321      321           
  Branches        2        2           
=======================================
  Hits          253      253           
  Misses         68       68

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[fedefernandez]

Nice refactor, looks good to me!