added integration test for virtual method inheritance

[johnperry-math]

Adds an integration test to address Issue #774

[adetaylor]

@johnperry-math I'd like to land this test now with an extra attribute:

#[ignore] // https://github.com/google/autocxx/issues/774

to avoid future merge conflicts, like the one which just occurred. Please could you update the PR to add that?

[johnperry-math]

Sorry it took so long. I believe I have made the change successfully and redid the pull request. I didn't mean to wait so long; again, sorry about that.

[adetaylor]

Thanks, but I fear you added the attribute to the wrong test!

[johnperry-math]

I'm so sorry. I dawdled so long that I had to spend a lot of time merging to reconcile our branches (e.g., the old tests.rs was gone) so I just searched on test_virtual and it didn't occur to me that there might be more than 1 function that started with that phrase. I've fixed it now (I even compared commands with the original issue to be sure).