Put back 'xblockutils/base_test.py'

[farhan]

The initiator the task is this PR
Parent Story: openedx/XBlock#675

As the community is using base_test.py for plain selenium integration tests that was removed, so let's put it back.

Since base_test.py is tightly coupled with workbench, so lets move it into the xblock-sdk repo and combine it with test_utils.py which seems to have related functionality as suggested here

[farhan]

xblockutils/base_test.py is extending from SeleniumTest class which is using bok-choy's WebAppTest class.

So it's linked to bok-choy library and I think we should keep it removed

@feanil what are your thoughts on it ?

[kdmccormick]

Ah, I thought that workbench.test.selenium_test.SeleniumTest was a third-party library. I didn't realize that it actually comes from xblock-sdk and uses bok-choy.

I support keeping it removed 👍🏻 Thanks for looking into that @farhan .