fix readytotest selector for IE testing

For some reason I don't understand yet, the xpath selector
  //*[@class='readytotest']
fails to match a <div class="readytotest"> in IE.  No match means
the browser test driver never thinks the browser is ready, so the
tests hang indefinitely.

This CL replaces that selenium By.xpath matcher with By.className,
which does work on IE.

//*[contains(@class,'foo')] does work, so those don't need to be
changed.

[kpreid2, 2012-12-05 20:29:52]

LGTM

https://codereview.appspot.com/6892044/diff/1/tests/com/google/caja/plugin/Br...
File tests/com/google/caja/plugin/BrowserTestCase.java (right):

https://codereview.appspot.com/6892044/diff/1/tests/com/google/caja/plugin/Br...
tests/com/google/caja/plugin/BrowserTestCase.java:278:
By.className("readytotest"));
This is (presumably) more correct anyway, as it will match one of many class
names.

[felix8a, 2012-12-05 20:33:52]

@r5177