We specify 1.13 in go.mod, so we should make sure it can be built and
tested on that version. Let's keep using the ioutil package for now,
since it was only deprecated from 1.16.
There was some code duplication between the go-junit-report binary, its
tests and the testdata/generate-golden script. This has been moved into
an internal package. The go-junit-report binary can now just focus on
flag parsing and validation, and it should be less likely that the
binary, tests and golden report generator behave differently.
