Add unless x != test to barrier guards

This tests that the following call to `foo bar` is guarded:

    unless bar != "bar"
      foo bar
    end

Author: hmac

ql/test/library-tests/dataflow/barrier-guards/barrier-guards.expected

@@ -1,3 +1,5 @@
-| barrier-guards.rb:3:4:3:15 | ... == ... | barrier-guards.rb:4:13:4:15 | foo | barrier-guards.rb:3:4:3:6 | foo | true |
-| barrier-guards.rb:9:4:9:24 | call to include? | barrier-guards.rb:10:13:10:15 | foo | barrier-guards.rb:9:21:9:23 | foo | true |
-| barrier-guards.rb:15:4:15:15 | ... != ... | barrier-guards.rb:18:14:18:16 | foo | barrier-guards.rb:15:4:15:6 | foo | false |
+| barrier-guards.rb:3:4:3:15 | ... == ... | barrier-guards.rb:4:5:4:7 | foo | barrier-guards.rb:3:4:3:6 | foo | true |
+| barrier-guards.rb:9:4:9:24 | call to include? | barrier-guards.rb:10:5:10:7 | foo | barrier-guards.rb:9:21:9:23 | foo | true |
+| barrier-guards.rb:15:4:15:15 | ... != ... | barrier-guards.rb:18:5:18:7 | foo | barrier-guards.rb:15:4:15:6 | foo | false |
+| barrier-guards.rb:21:8:21:19 | ... == ... | barrier-guards.rb:24:5:24:7 | foo | barrier-guards.rb:21:8:21:10 | foo | true |
+| barrier-guards.rb:27:8:27:19 | ... != ... | barrier-guards.rb:28:5:28:7 | foo | barrier-guards.rb:27:8:27:10 | foo | false |

ql/test/library-tests/dataflow/barrier-guards/barrier-guards.rb

@@ -1,22 +1,31 @@
 foo = "foo"
 
 if foo == "foo"
-    do_true foo
+    foo
 else
-    do_false foo
+    foo
 end
 
 if ["foo"].include?(foo)
-    do_true foo
+    foo
 else
-    do_false foo
+    foo
 end
 
 if foo != "foo"
-    do_true foo
+    foo
 else
-    do_false foo
+    foo
 end
 
-do_default foo
+unless foo == "foo"
+    foo
+else
+    foo
+end
+
+unless foo != "foo"
+    foo
+end
 
+foo