Default tests' data path to repo's data

My rough guess is that 99% of the times pdal_test is run, it uses ${CMAKE_SOURCE_DIR}/test/data as its data path. This patch sets the default test data path to that directory, using cmake's configure_file. Now you can just run:

```bash
pdal_test
```

to run the test suite.

The old invocations are still valid: `pdal_test /my/other/test/data`, etc.