tests: Add 317-nav-insert

[gadenbuie]

Tests rstudio/shiny#4179 and the additional cases the shiny-insert-tab message handler should cover in particular (from rstudio/shiny#1794 (comment)):

We need these test cases for anywhere we insert dynamic UI:

1. <script> blocks should run
2. <script> blocks should only run once
3. head()/singleton() should be respected
4. HTML widgets should work
   a. Even when the dependencies are not part of the initial page load
5. Shiny inputs/outputs should work
6. Subapps should work (include a shinyApp object right in the UI)

I also included a test that involves a custom element that modifies its attributes in its connected callback. For this kind of custom element, shiny-insert-tab would render the HTML twice, causing the final inserted UI not to match what the user expected. This was fixed by the linked shiny PR.