Update UI test expectations to accommodate shiny 1.6

[cpsievert]

In the next version of shiny (which we plan on submitting to CRAN in a week or so), we've made several changes to HTML/CSS markup that cause your unit tests to fail (in non-meaningful ways), and so we ask you to submit a new version of cytomapper to CRAN as soon as possible to address this issue.

This PR fixes the issue by updating your test expectations to work with shiny 1.6 and also adds skip_on_cran() to skip tests that make particularly strong assumptions about the HTML/CSS that shiny generates. This will make it so shiny can continue to make HTML/CSS changes as needed without requiring periscope to send updates to CRAN (you can still monitor the tests locally and/or on a CI service like Github Actions or Travis CI).

Let me know if you have any questions and please send a new version of periscope to CRAN with these updates to your tests as soon as possible.

[codecov]

Codecov Report

Merging #38 (3672fd3) into master (408ebf5) will increase coverage by 15.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master      #38       +/-   ##
===========================================
+ Coverage   60.40%   75.41%   +15.01%     
===========================================
  Files          12       12               
  Lines        1139     1139               
===========================================
+ Hits          688      859      +171     
+ Misses        451      280      -171     

Impacted Files	Coverage Δ
R/logger.R	91.17% <ø> (+67.22%)	⬆️
R/generate_template.R	100.00% <0.00%> (+2.32%)	⬆️
R/fw_helpers_external.R	95.23% <0.00%> (+2.38%)	⬆️
R/convert_template.R	100.00% <0.00%> (+5.40%)	⬆️

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[cb4ds]

Tested changes and approving since they are test-only changes. However note that Shiny 1.6 (RC branch) depends on development versions and non-cran packages so full testing will need to be done when Shiny 1.6 (and associated dependencies) make it onto CRAN.