mtest: simplify complete_exitcode
mtest: simplify complete_exitcode

There is no need anymore to pass the JUnit XML down to complete_exitcode.  Just
set self.junit in complete_gtest instead.

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