JavaScript: Add test case exposing two bugs in data flow library.

This test case exposes two bugs in our data flow library (fixed by the
two previous commits):

  - the charpreds of `SourcePathNode` and `SinkPathNode` only ensured
    that they were on a path from a source to a sink, not that they
    actually were the source/sink themselves;
  - function summarization would allow for non-level paths; in the
    test case, this meant that one of the summaries for `source`
    represented the path returning from `source` on line 13 and then
    flowing back into the call on line 15, in the process transforming
    the parity of the flow label and hence causing a spurious flow.

javascript/ql/test/library-tests/InterProceduralFlow/FlowLabels.expected

@@ -0,0 +1,2 @@
+| tst5.mjs:13:12:13:20 | source(0) | tst5.mjs:18:6:18:9 | flow |
+| tst5.mjs:15:8:15:19 | source(flow) | tst5.mjs:16:13:16:16 | flow |

javascript/ql/test/library-tests/InterProceduralFlow/FlowLabels.ql

@@ -0,0 +1,33 @@
+import javascript
+
+class Parity extends DataFlow::FlowLabel {
+  Parity() { this = "even" or this = "odd" }
+
+  Parity flip() { result != this }
+}
+
+class Config extends DataFlow::Configuration {
+  Config() { this = "config" }
+
+  override predicate isSource(DataFlow::Node nd, DataFlow::FlowLabel lbl) {
+    nd.(DataFlow::CallNode).getCalleeName() = "source" and
+    lbl = "even"
+  }
+
+  override predicate isSink(DataFlow::Node nd, DataFlow::FlowLabel lbl) {
+    nd = any(DataFlow::CallNode c | c.getCalleeName() = "sink").getAnArgument() and
+    lbl = "even"
+  }
+
+  override predicate isAdditionalFlowStep(DataFlow::Node pred,  DataFlow::Node succ, DataFlow::FlowLabel predLabel, DataFlow::FlowLabel succLabel) {
+    exists(DataFlow::CallNode c | c = succ |
+      c.getCalleeName() = "inc" and
+      pred = c.getAnArgument() and
+      succLabel = predLabel.(Parity).flip()
+    )
+  }
+}
+
+from Config cfg, DataFlow::PathNode source, DataFlow::PathNode sink
+where cfg.hasFlowPath(source, sink)
+select source, sink

javascript/ql/test/library-tests/InterProceduralFlow/tst5.mjs

@@ -0,0 +1,18 @@
+function source(x) {
+  return x;
+}
+
+function sink(x) {
+  return x;
+}
+
+function inc(x) {
+  return x+1;
+}
+
+var flow = source(0);  // source
+flow = inc(flow);
+flow = source(flow);   // source
+flow = sink(flow);     // sink for line 15, but not for line 13 (wrong parity)
+flow = inc(flow);
+sink(flow);            // sink for line 13, but not for line 15 (wrong parity)