gtk3 test: reduce variable scope
gtk3 test: use assert_equal instead of assert_nothing_raised

assert_nothing_raised does check anything. So we should not use it as
little as possible.
gtk3 test: remove duplicated information from test case

The parent test case class also has "GtkEntry" information.
gtk3 test: use object itself

Because the expected value should be the completion set by setter.
gtk3 test: reduce variable scope
gtk3 test: use search-changed signal for testing SearchEntry

It requires GTK+ 3.10 but I don't have 3.10 environment. So this test
may be broken. Sorry.