Skip tests failing on SL Chrome 71

[dreamofabear]

Temporary relief for #19647.

[rsimha]

Seems like there are 4 more failures: https://travis-ci.org/ampproject/amphtml/jobs/465047006#L698

[lannka]

I send a PR to see if the amp-pixel tests could be fixed: #19723

[dreamofabear]

[18:56:09] Safari 11.1.2 (Mac OS X 10.13.6): Executed 91 of 485 (Skipped 394) 2 FAILED
...
[18:57:07] Edge 17.17134.0 (Windows 10 0.0.0): Executed 90 of 485 (Skipped 395) 3 FAILED
...
[19:00:05] Chrome 71.0.3578 (Windows 10 0.0.0): Executed 313 of 485 (Skipped 169) 1 FAILED

[lannka]

@choumx very close! one more failure

DESCRIBE => amp-bind
  DESCRIBE => with [text] and [class]
    IT => "before each" hook for "should update CSS class when class binding changes"
      ✗ Error: Timeout waiting for All AMP components are laid out
          at poll (home/travis/build/ampproject/amphtml/testing/iframe.js:423:18)