[GLib] Enable tests for custom extension data type

[kou]

Describe the enhancement requested

There are some omitted tests in c_glib/test/test-extension-data-type.rb such as:

arrow/c_glib/test/test-extension-data-type.rb

Lines 53 to 56 in a3f3d8b

	def test_to_s
	omit("gobject-introspection gem doesn't support implementing methods for GLib object yet")
	data_type = UUIDDataType.new
	assert_equal("extension<uuid>", data_type.to_s)

We can enable them now because recent gobject-introspection gem can implement virtual methods.

Component(s)

GLib