unit tests: Check whether pytest-xdist is available
unit tests: Check whether pytest-xdist is available

Also, always run pytest using the module format to ensure that we're
always using the Python interpreter selected by the user.

Fixes issue reported in https://github.com/mesonbuild/meson/pull/5621#issuecomment-513034198

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