[reference-bindings] Disable a test when we backdeploy.

This is an experimental feature that is not being actively worked on. Disable
the test when we backwards deploy for now to fix the bots. I am not deleting the
test since it seems to be working on the main bots and it makes sense to at
least keep that going to prevent further breakage.

rdar://159026031

Author: gottesmm

test/Interpreter/reference_bindings.swift

@@ -2,6 +2,7 @@
 
 // REQUIRES: executable_test
 // REQUIRES: swift_feature_ReferenceBindings
+// UNSUPPORTED: back_deployment_runtime
 
 import StdlibUnittest