Get `x.py test` output in human-readable format

[alexreg]

When x.py test fails, the stdout and stderr are in JSON format, most annoyingly. How can we get it in the usual human-readable format? (This should really be default, by the way.)

[Centril]

Maybe it would be sufficient to pretty print the JSON for now?

[phansch]

I think #59128 may be fixing this, in addition to colored output? Specifically 7c8482c.

cc @oli-obk

[alexreg]

@phansch Cool. You may well be right, judging by that. Incidentally, I should have said the --error-format option is misleading (and not properly documented in the case of x.py)... it seems to simply get ignored.

[jyn514]

x test no longer outputs in JSON format by default; I'm going to close this.