mtest: include classname in <testcase> JUnit element
mtest: include classname in <testcase> JUnit element

It looks like GitLab ignores the suite name and actually uses
the classname.  Adjust the output accordingly.

Fixes: #8316
Signed-off-by: Paolo Bonzini <69d3ebcf5ee4e7f05a50dc23cd96655b730804b4@redhat.com>

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