fix: trim input and output correctly

[merceyz]

What:
Fixed babel-plugin-tester not trimming strings correctly

Why:
When using a formatter that adds an empty line at the end of the string babel-plugin-tester will save the result with this empty line. In subsequent runs, when reading the output file it trims it off, causing the strings to not match.

How:
Call trim at assert.equal for both values and don't trim code passed to formatResult

[codecov-io]

Codecov Report

Merging #44 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #44   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines         129    129           
  Branches       32     32           
=====================================
  Hits          129    129

Impacted Files	Coverage Δ
src/index.js	100% <100%> (ø)	⬆️

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