Reorganization of pgtap files for contraction

[cvvergara]

Because contraction was the only function the structure of the contraction sub-directory on pgtap is:
only showing directories:

contraction
├── combined
├── deadend
│   ├── directed
│   └── undirected
├── edge_cases
└── linear
    ├── directed
    └── undirected

The structure should change to

contraction
└── contraction
    └── edge_cases
        ├── combined
        ├── deadend
        │   ├── directed
        │   └── undirected
        └── linear
            ├── directed
            └── undirected

Therefore when the new functions are added the top two levels of the directory will look as follows

contraction
├── contraction
│  └── edge_cases
└── contractionDeadEnd
│  └── edge_cases
└── contractionLinear
   └── edge_cases