mtest: improve JUnit XML generation for TAP testsuites
mtest: improve JUnit XML generation for TAP testsuites

Include the names from the TAP output and the SKIP/TODO explanations
if present.  Omit the classname attribute, it is optional.

In order to enable this, TestRun.results becomes a list of TAPParser.Test
objects.  If in the future there are other kinds of subtest results a
new class can be introduced, but for now it is enough.

Signed-off-by: Paolo Bonzini <69d3ebcf5ee4e7f05a50dc23cd96655b730804b4@redhat.com>

pexip/meson,jpakkane/meson,jpakkane/meson,mesonbuild/meson,mesonbuild/meson,QuLogic/meson,QuLogic/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,jpakkane/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,jpakkane/meson,jpakkane/meson,pexip/meson,mesonbuild/meson,QuLogic/meson,QuLogic/meson,jpakkane/meson,pexip/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,pexip/meson,jpakkane/meson,QuLogic/meson,QuLogic/meson