Make result tables more readable

[Psychpsyo]

Description

This makes the first column in tables of test results alternate between white and light gray for the background color of its rows, making it easier to see which test scores line up with which test.
The backgrounds for the scores themselves are left unaltered, as those have actual meaning.

Review Information

Personally, I wrote this css into the inline stylesheet on the production site (can use this page to test) and then copy-pasted it into that same inline stylesheet in the code, using the Github web-editor.
If I were reviewing this, I'd probably just do the reverse, as that sounds more straight-forward than cloning this into a dev environment and shows the change just as well.

Changes

I added a single new CSS rule in the inline stylesheet in wpt-results.js.
The rule looks like this:

tr:nth-of-type(2n) td:first-child {
    background-color: var(--paper-grey-100);
}

2n conveniently starts selecting on the second row, so that the headers don't get styled.

Requirements

You need to be able to look at the results of any test, with this rule inserted into the stylesheet.

[past]

#4089 already handled this. Closing this PR as a duplicate, but happy to look at other PRs if there are improvements to what landed.