[GTK] Fix tests
[GTK] Fix tests

The `notify:visible` signal is deceptive; it gets fired even if main
loop is not running. Adding it to be called on the main loop via
idle_add seems to solve the issue.

shivaprsdv/pywebview,r0x0r/pywebview,r0x0r/pywebview,r0x0r/pywebview,shivaprsdv/pywebview,r0x0r/pywebview,shivaprsdv/pywebview,r0x0r/pywebview,shivaprsdv/pywebview