Fix API tests for geckodriver 0.23 & ChromeDriver 2.41

[scottlowe]

I have fixed the broken browser API tests for recent versions of Firefox, Geckodriver, Chrome & ChromeDriver.

This PR partially addresses issue #162.

Versions

My test system used the following dependency versions:

• Firefox 62.0.3
• Geckodriver 0.23.0
• Chrome 70.0.3538.67
• ChromeDriver 2.41.578706

OS: macOS Mojave.

ETAOIN_TEST_DRIVERS="[:chrome :firefox]" lein test

# ...

Ran 72 tests containing 183 assertions.
0 failures, 0 errors.

I have not yet been able run the tests for the :phantomjs and :safari drivers, so I would expect those to fail - however, I hope this PR gets you closer to getting the full test suite green.

Removal of Firefox set-cookie test

Firefox (via Marionette) no longer allows cookies to be set against a domain using the file:/// protocol. It only allows setting cookies against a network scheme. The test will fail with a Document is cookie-averse exception from Marionette.

Change to delete-session

Please note that I have also made a change that allows for delete-session to be called against an already deleted session.

I made this change because the API test fixture uses with-driver which calls quit when a test has completed. quit also calls delete-session behind the scenes. This caused an exception in the close-window test, which also calls delete-session.

This change is perhaps debatable and I could make another PR without this change if it is not desirable, but we would then need to come up with an alternative approach to fixing the failing test-close-window test, which this change addresses.