Add tests for `matchers.py`

[HiromuHota]

Description of the problems or issues

Is your pull request related to a problem? Please describe.

Towards #380.

Does your pull request fix any issue.

Towards #380.

Description of the proposed changes

This PR adds more tests.

Test plan

This PR adds more tests.

Checklist

• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.
• [ ] I have updated the CHANGELOG.rst accordingly.

[codecov-commenter]

Codecov Report

Merging #472 into master will increase coverage by 1.49%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #472      +/-   ##
==========================================
+ Coverage   84.33%   85.82%   +1.49%     
==========================================
  Files          88       88              
  Lines        4557     4557              
  Branches      847      847              
==========================================
+ Hits         3843     3911      +68     
+ Misses        527      464      -63     
+ Partials      187      182       -5     

Flag	Coverage Δ
#unittests	85.82% <ø> (+1.49%)	:arrow_up:

Impacted Files	Coverage Δ
src/fonduer/candidates/models/span_mention.py	82.24% <0.00%> (+7.47%)	:arrow_up:
src/fonduer/candidates/matchers.py	97.31% <0.00%> (+32.25%)	:arrow_up: