Moved test suite over to Kotlin.

[psh]

Description

Related to the discussion on issue #1389, this PR moves the Java unit test suite over to Kotlin.

Changes of note:

• Updated the version of Kotlin to match the latest release (note: Kotlin is still only a test dependency, so not enabled for use in the production codebase)
• Slight change of order in the build.gradle dependencies to tidy things up.
• Utilized the more idiomatic Mockito-Kotlin (MIT licensed) in tests.

Tests performed

All unit tests, across all build variants, run cleanly.

[codecov-io]

Codecov Report

Merging #1428 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1428   +/-   ##
======================================
  Coverage     3.3%    3.3%           
======================================
  Files         128     128           
  Lines        6839    6839           
  Branches      669     669           
======================================
  Hits          226     226           
  Misses       6598    6598           
  Partials       15      15

Impacted Files	Coverage Δ
...java/fr/free/nrw/commons/category/CategoryDao.java	0% <ø> (ø)	⬆️

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[misaochan]

@psh Wonderful, thank you! :) Looks good to me in general, but it would be good if someone familiar with Kotlin (@maskaravivek ?) could review this before we merge.

[maskaravivek]

Looks good to me as well. The PR simply converts Java code to Kotlin. As long as the checks are passing, it should be okay. :)

[misaochan]

Thanks all! Would be great if we could get a section in the wiki about this, also. :)