validate:launcher: Make failure as <failure> in our xunit reporter

I think it was a mistake to call them <error> as the two notions are
different (we marked failed test as "failures" in the <testuite> node).

Should make gitlab happy with our file!
4 jobs for fix_xunit_files in 6 minutes and 8 seconds (queued for 1 minute and 32 seconds)
Status Job ID Name Coverage
  Preparation
passed #58084
gst indent

00:00:17

passed #58083
manifest

00:00:21

 
  Build
passed #58085
build fedora x86_64

00:02:02

 
  Full Builds
passed #58086
build android arm64 api28

00:03:26