Properly tested config detection

[DanielaSfregola]

Although this lowers down the test coverage, it makes tests independent from the env variables set up on the machine when running tests.

[codecov]

Codecov Report

Merging #117 into master will increase coverage by -0.94%.

@@            Coverage Diff             @@
##           master     #117      +/-   ##
==========================================
- Coverage   93.37%   92.44%   -0.94%     
==========================================
  Files          60       60              
  Lines        1057     1059       +2     
  Branches       12       11       -1     
==========================================
- Hits          987      979       -8     
- Misses         70       80      +10

Impacted Files	Coverage Δ
...clients/rest/favorites/TwitterFavoriteClient.scala	100% <ø> (ø)	✅
.../danielasfregola/twitter4s/TwitterRestClient.scala	0% <ø> (-100%)	❌
...elasfregola/twitter4s/TwitterStreamingClient.scala	0% <ø> (-100%)	❌
...anielasfregola/twitter4s/util/Configurations.scala	66.66% <50%> (+16.66%)	✅

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[DanielaSfregola]

LGTM