compile-test fails to detect extranous errors on windows

[oli-obk]

the problem is in this function: https://github.com/rust-lang/rust/blob/master/src/compiletest/runtest.rs#L1009

since windows-paths contain a colon, finding the error/warning through counting colons is not the way to go.

i need this fixed, so i'm fixing it.
should i use regex or simply check if the line starts with the testfile-name, and skip those chars before counting colons?

[alexcrichton]

I believe this was fixed in #27974