• wptrunner: detect solid color screenshots for failing reftests 

    The output looks like this:

```
  ▶ FAIL [expected PASS] /css/CSS2/box-display/root-box-003.xht
  │   → /css/CSS2/box-display/root-box-003.xht 54a9df64f1476dd12020019d7cf22ac34d727bc0
  │   → /css/CSS2/box-display/root-box-003-ref.xht 636eb693bc214b6e1c64e6566c48e69e6777b946
  └   → Screenshot is solid color 0xFFFFFF for /css/CSS2/box-display/root-box-003.xht
```
    @SimonSapin
    SimonSapin committed Dec 3, 2019
    8dc703f