Move substitution test

[rtaupe]

Move SubstitutionTest and UnifierTest to common package because Substitution and Unifier are also in common package. Move also two static methods from NaiveGrounder to a new class SubstitutionTestUtil because those methods are only used by tests.

[codecov]

Codecov Report

Merging #247 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #247      +/-   ##
============================================
- Coverage     76.83%   76.78%   -0.05%     
+ Complexity     2056     2051       -5     
============================================
  Files           150      150              
  Lines          6782     6764      -18     
  Branches       1184     1180       -4     
============================================
- Hits           5211     5194      -17     
  Misses         1177     1177              
+ Partials        394      393       -1     

Impacted Files	Coverage Δ	Complexity Δ
.../at/ac/tuwien/kr/alpha/grounder/NaiveGrounder.java	87.46% <ø> (-0.37%)	101.00 <0.00> (-5.00)

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