Add an optimizer switch for enabling the hypergraph optimizer.
The hypergraph optimizer, being experimental, is allowed to
refuse queries with a special error code saying “we do not support
this yet”; currently, this holds for every single query.
Give MTR an --hypergraph flag to enable this optimizer switch
at the start.

In order to facilitate testing, MTR will take this error and
treat it as non-fatal when in --hypergraph mode. Furthermore,
if the diff solely consists of result lines that are replaced
by such errors, the test is treated as skipped. This allows
running the entire test suite under --hypergraph, and getting
no errors as long as everything hypergraph-suppored returns
the right answer.

Change-Id: I9a8b7a6a88651f44d2c57ecfa7633e7a9fdb53c5