mtest: tweak the gathering of stdo_task/stde_task results
mtest: tweak the gathering of stdo_task/stde_task results

After the next patch, we will need to complete parse_task before
stdo_task (because parse_task will not set the "stdo" variable
anymore but it will still collect stdout just like now).  Do
the change now to isolate the more complicated changes.

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