tests: Add GStreamer library directory to DYLD_LIBRARY_PATH (#31163)

This fixes an issue where the dylib for harfbuzz cannot be found when
running unit tests on some systems, because unit tests don't get their
rpaths adjusted during build. This is quite likely an issue with dylib
dependency management. We just need a bit more exploration of how this
is traditionally handled.

python/servo/platform/linux.py

@@ -82,9 +82,6 @@ class Linux(Base):
         self.is_linux = True
         (self.distro, self.version) = Linux.get_distro_and_version()
 
-    def library_path_variable_name(self):
-        return "LD_LIBRARY_PATH"
-
     @staticmethod
     def get_distro_and_version() -> Tuple[str, str]:
         distrib = distro.name()