Improve labelling for plot_quality()

[mdingemanse]

Simple commit and updated test plot to address #116, seems ready for review

[mdingemanse]

See this issue for something I'm running into with tests

[mdingemanse]

@PabRod i think the failing tests here on this PR have to do with the same issue as #118 and I would like to resolve it so that i can produce PRs that do not fail tests — any insights?

[PabRod]

The problem is that the snapshot seems to be different than expected. It might be related to #118, but I'm not 100% sure. I'll try it in my laptop as soon as possible.

  ══ Failed tests ════════════════════════════════════════════════════════════════
  ── Failure ('test-plots.R:53:3'): Plot quality ─────────────────────────────────
  Snapshot of `plot_quality_with_save(testdata)` to 'plots/plot_quality.png' has changed
  * Download and unzip run artifact
  * Copy 'tests/testthat/_snaps/plots/plot_quality.new.png' to local test directory
  * Run `testthat::snapshot_review('plots/')` to review changes