test: skip terminfo parsing in Miri
Terminfo parsing takes a significant amount of time in Miri, making libtest startup very slow. To work around that Miri in fact unsets the `TERM` variable. However, this means we don't get colors in `cargo miri test`.
So I propose we add some logic in libtest that skips parsing terminfo files under Miri, and just uses the regular basic coloring commands (taken from the `colored` crate).
As far as I can see, these two commands are all that libtest ever needs from terminfo, so Miri doesn't even lose any functionality through this. If you want I can entirely remove the terminfo parsing code and just use these commands instead.
Cc https://github.com/rust-lang/miri/issues/2292 `@saethlin`
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