junitxml: compile a regex lazily
junitxml: compile a regex lazily

Instead of slowing down startup, and making the code harder to follow,
compile it lazily (it is still cached internally).

nicoddemus/pytest,nicoddemus/pytest,The-Compiler/pytest,The-Compiler/pytest,pytest-dev/pytest,RonnyPfannschmidt/pytest,markshao/pytest,Akasurde/pytest