CSS variables tests should require trimming values

[mgol]

There was a spec change a few years ago requiring spaces around CSS declaration's value to be trimmed: https://github.com/w3c/csswg-drafts/pull/1986. The discussion leading to this spec change: https://github.com/w3c/csswg-drafts/issues/774.

The PR is marked as tested but the CSS Variable tests at https://github.com/web-platform-tests/wpt/blob/246b1bbb31f5cfa50dcd9cf0e91a14b32d9faf4e/css/css-variables/variable-definition.html don't seem to account for that. For example, the trailing white space (single space) test requires the following input: "--var:value " to result in the value "value " - note the trailing space.