according to a poll of gay people in my phone, purple is the most popular color to use for highlighting

| color      | percentage |
| ---------- | ---------- |
| bold white | 6%         |
| blue       | 14%        |
| cyan       | 26%        |
| purple     | 37%        |
| magenta    | 17%        |

unfortunately, purple is not supported by 16-color terminals, which rustc apparently wants to support for some reason.
until we require support for full 256-color terms (e.g. by doing the same feature detection as we currently do for urls), we can't use it.

instead, i have collapsed the purple votes into magenta on the theory that they're close, and also because magenta is pretty.

this is kinda jank because it means people need both machines to bless the tests