libtest: Only colorize output if stdout is a tty

[bnoordhuis]

Fixes #14570.

[sfackler]

It'd be nice to add a flag to force colored output too but I don't know if that needs to block this.

Now that I think about it, since test output is captured now, we may be able to remove the single task requirement as well.

[bnoordhuis]

It'd be nice to add a flag to force colored output too but I don't know if that needs to block this.

I've been thinking the same thing. I'll try to follow up with a patch later this week.

Now that I think about it, since test output is captured now, we may be able to remove the single task requirement as well.

I'm not sure what you mean. Do you want to remove RUST_TEST_TASKS?

[sfackler]

RUST_TEST_TASKS would stay, but use_color wouldn't check it. We used to because there was no synchronization of the test result output which meant that colored output would break everything. That's no longer the case, so we should be able to color output no matter what.

For the flag, it'd probably make the most sense to mirror rustc's:

    --color auto|always|never
                        Configure coloring of output: auto = colorize, if
                        output goes to a tty (default); always = always
                        colorize output; never = never colorize output