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.
Merge pull request #466 from gadomski/run-tests-without-path

Default tests' data path to repo's data