chromedriver doesn't allow changing Object.prototype to add enumerable
properties, but this test requires setting some values on
Object.prototype.  When Object.prototype.a is set to:

  {b: {c: 'on proto'}}

chromedriver fails with:

    JavascriptErrorException: javascript error (500): Maximum call stack size exceeded
      (Session info: chrome=72.0.3626.121)

    Remote-end stacktrace:

    #0 0x563ff3a32a59 <unknown>
    #1 0x563ff39cb7f3 <unknown>
    #2 0x563ff38fcd7c <unknown>
    #3 0x563ff38ff78c <unknown>
    #4 0x563ff38ff5f7 <unknown>
    #5 0x563ff38ffbe7 <unknown>
    #6 0x563ff38fff1b <unknown>
    #7 0x563ff38a3f7a <unknown>
    #8 0x563ff3899bf2 <unknown>
    #9 0x563ff38a37b7 <unknown>
    #10 0x563ff3899ac3 <unknown>
    #11 0x563ff38782d2 <unknown>
    #12 0x563ff3879112 <unknown>
    #13 0x563ff39fe865 <unknown>
    #14 0x563ff39ff32b <unknown>
    #15 0x563ff39ff70c <unknown>
    #16 0x563ff39d940a <unknown>
    #17 0x563ff39ff997 <unknown>
    #18 0x563ff39e9947 <unknown>
    #19 0x563ff3a1a800 <unknown>
    #20 0x563ff3a3c8be <unknown>
    #21 0x7f3bf4545494 start_thread
    #22 0x7f3bf2d58a8f clone

    Ran 1 tests finished in 2.0 seconds.
      • 0 ran as expected. 0 tests skipped.
      • 1 tests had errors unexpectedly

Work around this problem by cleaning up the test environment so
Object.prototype no longer has the override by the time chromedriver
tries to inspect the test result.

While here, fix the other tests to use the t.add_cleanup() function
so they'll cleanup their test environment in case they exit in
some other way besides reaching t.done().

The underlying chromedriver issue is tracked upstream at
https://crbug.com/chromedriver/2555.

Bug: 934844
Change-Id: Id1b4ab2a908bfbc001e2a2d045eeec3ef01c24d9