Make tests able to import local files

[stephan-nordnes-eriksen]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Bugfix:

With this fix, the local imports in tests are resolved, and you can now do the following inside your tests;

import { test } from 'ava'
import { MyModule } from './myModule'
import { MyOtherModule } from '../whatever/myOtherModule'

// Use MyModule in tests

• What is the current behavior? (You can also link to an open issue here)
  Currently, the tests are not able to import files locally, effectively limiting unit testing to only testing exposed code

The following does not work;

import { test } from 'ava'
import { MyModule } from './myModule'
import { MyOtherModule } from '../whatever/myOtherModule'

// Use MyModule in tests

• What is the new behavior (if this is a feature change)?
  I consider this a bugfix.

• Other information:
  Resolves How to execute tests for internal methods? #49

[codecov]

Codecov Report

Merging #58 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #58   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines          18     18           
=====================================
  Hits           18     18

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[bitjson]

Thanks for the pull request @stephan-nordnes-eriksen! Sorry to pull a conflict over it. If you're still interested, would you mind merging?