Ruby: Add (failing) test case for flow out via assignment expression.

Author: michaelnebel

ruby/ql/test/library-tests/dataflow/global/instance_variables.rb

@@ -77,3 +77,8 @@ def set_field_on x
 foo12 = Foo.new
 set_field_on (foo12) # space after `set_field_on` is important for this test
 sink(foo12.get_field) # $ hasValueFlow=28
+
+foo13 = Foo.new
+foo14 = Foo.new
+set_field_on(foo14 = foo13)
+sink(foo13.get_field) # $ hasValueFlow=28