PS: Add local-flow test.

Author: MathiasVP

powershell/ql/test/library-tests/dataflow/local/test.expected

@@ -0,0 +1,8 @@
+$a1 = Source()
+Sink($a1)
+
+$b = GetBool()
+if($b) {
+    $a2 = Source()
+}
+Sink($a2)

powershell/ql/test/library-tests/dataflow/local/test.ps1

@@ -0,0 +1,6 @@
+import powershell
+import semmle.code.powershell.dataflow.DataFlow
+
+from DataFlow::Node pred, DataFlow::Node succ
+where DataFlow::localFlowStep(pred, succ)
+select pred, succ