Add demo: Physics Tests 2D

Similar to its 3D counterpart, with tests using 2D physics.

2d/physics_tests/utils/label_test.gd

@@ -0,0 +1,13 @@
+extends Label
+
+
+var test_name setget _set_test_name
+
+
+func _ready():
+	set_text("Select a test from the menu to start it")
+
+
+func _set_test_name(value):
+	test_name = value
+	set_text("Test: %s" % test_name)