[lit] Remove ANSI control characters from xunit output

Failing test output sometimes contains control characters like \x1b (e.g.
if there was some -fcolor-diagnostics output) which are not allowed inside
XML files. This causes problems with CI systems: for example, the Jenkins
JUnit XML will throw an exception when ecountering those characters and
similar problems also occur with GitLab CI.

Reviewed By: yln, jdenny

Differential Revision: https://reviews.llvm.org/D84233