tests: fix test_meson_installed after the recent appveyor image update
tests: fix test_meson_installed after the recent appveyor image update

Since the last appveyor image update which also included a msys2 update
test_meson_installed() fails for the msys2 mingw jobs complaining that
the install path isn't included in PYTHONPATH.

It complains that "site-packages\" isn't included in "site-packages" ignoring
that the separator should be ignored here. Work around the issue by making
sure that the path set as PYTHONPATH always ends with os.sep.

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