fix: allow JS test to run after shutdown test

shutdown notebook test closes casper browser, here we re-instatiate it
after the test
don't delete casper instance on shutdown test

a lunch conversation with @minrk and @takluyver lead me to just make a
popup and have it be closed, instead of having to reorder the test suite
so that this test always runs last. Now it doesn't matter what order the
test suite runs in, and gives us a pattern for testing window closing
behavior should we need it elsewhere.
Merge pull request #4451 from ivanov/shutdown-js

fix: allow JS test to run after shutdown test