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.
