Initial shinytest tests for themable flexdashboard

[MadhulikaTanuboddi]

Added shinytest tests for the recent bslib/thematic theming available for flexdashboard.

Ref: https://pkgs.rstudio.com/flexdashboard/articles/theme.html

Individual app/test directories are added and currently covering the following scenarios. Note that in the Rmarkdown output format, we had to specify the bootstrap version to 4. This is because bootstrap 5 does not work well with shinytest. This is a know limitation and Barret is working on shinytest2 to solve this problem

1. Default or no version specified so that the bootstrap 5 would be the default version. However, due to above limitation, temporarily we set the version to 4
2. Testing one bootswatch theme. In this case darkly
3. Testing custom settings i.e primary colors, google fonts etc.

TODO:

• [ ] Storyboard tab -- Find a way to click on the "Interactive Plots" storyboard tab
• [x] Gauge meters animation changes everytime shinytest is run. Fix this. Probably add Sys.sleep. -> Update. Talked to Barret and he confirmed adding sleep is the decent solution to wait for the animations to be done. This change is made in https://github.com/rstudio/shinycoreci-apps/pull/197/commits/8801ac05d5b3a94564b276be4ab5a4218c9a7b36

cc: @shalutiwari

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