"A more real AMPHTML image ad" test flaky in INABOX_LITE experiment

Integration tests in `test-amphtml-ads.js` are flaky on Headless Chrome in experiment B (INABOX_LITE):
```
DESCRIBE => A more real AMPHTML image ad
DESCRIBE => ATF within friendly frame or safe frame
DESCRIBE =>
IT => should properly render ad in a friendly iframe with viewability pings
✗ Error: Timeout of 15000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.
●
IT => should properly render ad in a safe frame with viewability pings
✗ Error: Timeout of 15000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.
●
DESCRIBE => No Host Script within friendly frame or safe frame
DESCRIBE =>
IT => should properly render ad in a friendly iframe with viewability pings
✗ Error: Timeout of 15000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.
●
IT => should properly render ad in a safe frame with viewability pings
✗ Error: Timeout of 15000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.

[23:25:01] HeadlessChrome 76.0.3809 (Linux 0.0.0): Executed 266 of 440 (Skipped 174) 4 FAILED
```

Two flakes on master today.

https://travis-ci.org/ampproject/amphtml/builds/582846281
https://travis-ci.org/ampproject/amphtml/builds/582910186

/to @lannka