Fix tgffileformat test: make unidirectional the first edge type
- by adding a new edge type in 8c6f0eac73b0fa63a8ee56c1acb091ac7167e2f9 and making the default bidirectional, some tests were broken (which assume that the first edge type is unidirectional).
Swap the two around, although that still seems like a hack. Tests
that need unidirectional edges should get them explicitly.