Add test for erb flow

ruby/ql/test/library-tests/dataflow/erb/erb.expected

@@ -0,0 +1,5 @@
+testFailures
+edges
+nodes
+subpaths
+#select

ruby/ql/test/library-tests/dataflow/erb/erb.ql

@@ -0,0 +1,13 @@
+/**
+ * @kind path-problem
+ */
+
+import codeql.ruby.AST
+import codeql.ruby.CFG
+import TestUtilities.InlineFlowTest
+import ValueFlowTest<DefaultFlowConfig>
+import ValueFlow::PathGraph
+
+from ValueFlow::PathNode source, ValueFlow::PathNode sink
+where ValueFlow::flowPath(source, sink)
+select sink, source, sink, "$@", source, source.toString()

ruby/ql/test/library-tests/dataflow/erb/main.rb

@@ -0,0 +1,7 @@
+class App
+    def run
+        x = source(1)
+        view = View.new(x)
+        render(view)
+    end
+end

ruby/ql/test/library-tests/dataflow/erb/view.erb

@@ -0,0 +1 @@
+<%= foo() %>

ruby/ql/test/library-tests/dataflow/erb/view.rb

@@ -0,0 +1,9 @@
+class View
+    def initialize(x)
+        @x = x
+    end
+
+    def foo
+        sink(@x) # $ hasValueFlow=1
+    end
+end