Package extension for JET.jl

[jakobjpeters]

JET.jl test failures from JET.test_package are not handled, throwing this error:

ERROR: MethodError: no method matching to_xml(::JET.JETTestFailure)
The function `to_xml` exists, but no method is defined for this combination of argument types.
Closest candidates are:
  to_xml(::Test.Pass)
   @ TestReports ~/.julia/packages/TestReports/zj3uX/src/to_xml.jl:225
  to_xml(::Test.Fail)
   @ TestReports ~/.julia/packages/TestReports/zj3uX/src/to_xml.jl:236
  to_xml(::Test.Broken)
   @ TestReports ~/.julia/packages/TestReports/zj3uX/src/to_xml.jl:242

It would be great to make a package extension to define this method.