unit tests: Don't use pytest when running single tests
unit tests: Don't use pytest when running single tests

On my machine this spawns 24 processes and then runs like the single
test I asked it to run. With this change, running a single test goes
from 7 seconds to less than a second.

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