run_unittests: fix undefined variable in error message
run_unittests: fix undefined variable in error message

In commit fe973d9fc45581f20fefc41fc0b8eb0066c0129d, some uses of p got
rewritten to compiler.language, but not all. We'd still raise an error
message, but for the wrong thing. o_O

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