Test multiple transitions on same element

It is most likely that multiple transitions on the same element won't work correctly when transitions are reversed.

Example code:

```
.some-element {
color: red;
left: 0;
transition: color 1s, left 2s;
}

.some-element.animate-in {
color: blue;
left: 50%;
}
```

No source files or tests are named in the issue. Start by locating the transition handling and existing animation tests, then reproduce the supplied CSS example with color and left transitions while reversing the animation. Done means a regression test confirms multiple transitions on one element reverse correctly.

Written by the indexing model from the issue text.