component/firefox: discard stderr output in firefox-slapos.

background: firefox 51 outputs non-ASCII character when it is terminated and current erp5.util cannot recognize well if the test finishes.
this commit is a workaround for this issue, so that we can use firefox 51 without touching ERP5TypeFunctionalTestCase, that is important to test old ERP5 revision.

also quote arguments.

component/firefox/buildout.cfg

@@ -94,7 +94,8 @@ script =
   export LD_LIBRARY_PATH="$LD_LIBRARY_PATH:${xz-utils:location}/lib"
   export LD_LIBRARY_PATH="$LD_LIBRARY_PATH:${zlib:location}/lib"
   export PATH=${fontconfig:location}/bin:$PATH
-  exec %(location)s/firefox $*""")
+  exec %(location)s/firefox "$@" 2> /dev/null
+  """)
   wrapper.close()
   os.chmod(wrapper_location, 0755)