Added unit tests for gnomer-color-palette and plotting functions

[toumban1]

What changes are proposed in this pull request? Unit tests were added for the gnomer-color-palette functions and plotting functions in mutation_viz.

If there is an GitHub issue associated with this pull request, please provide link.

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Reviewer Checklist (if item does not apply, mark is as complete)

• [x] PR branch has pulled the most recent updates from main branch. Ensure the pull request branch and your local version match and both have the latest updates from the main branch.
• [x] If a new function was added, function included in _pkgdown.yml
• [x] If a bug was fixed, a unit test was added for the bug check
• [x] Run pkgdown::build_site(). Check the R console for errors, and review the rendered website.
• [x] Code coverage is suitable for any new functions/features. Review coverage with withr::with_envvar(new = c("NOT_CRAN" = "true"), covr::report()). Begin in a fresh R session without any packages loaded.
• [x] R CMD Check runs without errors, warnings, and notes
• [x] usethis::use_spell_check() runs with no spelling errors in documentation

When the branch is ready to be merged into master:

• [x] Update NEWS.md with the changes from this pull request under the heading "# cbioportalR (development version)". If there is an issue associated with the pull request, reference it in parentheses at the end update (see NEWS.md for examples).
  • [x] Run codemetar::write_codemeta()
  • [x] Run usethis::use_spell_check() again
  • [x] Approve Pull Request
  • [ ] Merge the PR

[karissawhiting]

Let's make sure we are doing the right thing with the large image files. We may want to gitignore, but look at snapshot test that docs to make sure best practices.

[hfuchs5]

I'm adding an eval statement to the plots tests and the .gitignore will have .svg on the list.