stderr from ui tests is being generated to wrong place (recent regression)

[pnkfelix]

I think I messed something up in #49900. Some work I've been doing locally has indicated that each time I run compiletest with the ui test suite, it is overwriting the expected stderr files in the source tree. (oops!)

sorry about that.

Will put up PR to fix that pronto.

[pnkfelix]

(filed #50116 since the docs could have helped me avoid my mistake here.)