gtk/tests: Fix for -Wmissing-declarations

gtk/tests: Fix for -Wmissing-declarations

I'm adding a bunch of fixes for gcc complaining about
-Wmissing-declarations after finding a bunch of cases today where I
had forgotten to make functions static in the CSS code.

This patch fixes the tests in gtk/tests.

After this last patch, the gtk/ subdir should now compile without
warnings when this flag is enabled.