Add testing infrastructure for anywidgets, plus test for layer_manager and layer_manager_row

[sufyanAbbasi]

• Adds testing infrastructure using Karma/Jasmine to run tests for the new anywidgets.
• These can be invoked using npm run test to build and run all tests, or
• npm run test:watch to run the tests continuously.
  • Note that you must also run npm run dev in a separate terminal for the dev server to update the files.
• A new browser Chrome browser window will open and start executing the tests.
• Adds tests for layer_manager and layer_manager_row components.
• Rearranges some CSS classes for slightly better testability.

[naschmitz]

Looks great! Just a few minor comments.