run_unittests.sh: fix Python DeprecationWarning
run_unittests.sh: fix Python DeprecationWarning

Fix the following Python error:

  D:\a\1\s\run_unittests.py:6654: DeprecationWarning: invalid escape sequence \
    self.assertEqual(libhello_nolib.get_pkgconfig_variable(escaped_var, {}), hello world)

Use a raw string literal.

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