amp-base-carousel e2e test is flaky

```
1) AMP carousel arrows with custom arrows
firefox
Standalone environment

should go to the next slide:
AssertionError: expected { Object (bottom, height, ...) } to have property 'x' of 0, but got 800
+ expected - actual
-800
+0
```

Saw the above failures a couple of times this week. It has always been in firefox.
https://travis-ci.org/ampproject/amphtml/jobs/576131623

Start with the amp-base-carousel end-to-end test and the linked Travis CI Firefox failure, focusing on the assertion that expects the next slide's x position to be 0. Reproduce the failure in Firefox and inspect the test's timing or navigation behavior. Done means the test reliably passes for the custom-arrow next-slide case in Firefox.

Written by the indexing model from the issue text.