PS: Add tests.

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

@@ -0,0 +1,6 @@
+models
+edges
+nodes
+subpaths
+testFailures
+#select

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

@@ -0,0 +1,6 @@
+$a.f = Source "1"
+Sink $a.f # $ MISSING: hasValueFlow=1
+
+$a.f = Source "2"
+$a.f = 0
+Sink $a.f # clean

powershell/ql/test/library-tests/dataflow/fields/test.ql

@@ -0,0 +1,13 @@
+/**
+ * @kind path-problem
+ */
+
+import powershell
+import semmle.code.powershell.dataflow.DataFlow
+private import TestUtilities.InlineFlowTest
+import DefaultFlowTest
+import ValueFlow::PathGraph
+
+from ValueFlow::PathNode source, ValueFlow::PathNode sink
+where ValueFlow::flowPath(source, sink)
+select sink, source, sink, "$@", source, source.toString()