Cleaning up BlackBoxSourceHelper - use absolute file paths. (#789)

* Cleaning up BlackBoxSourceHelper - use absolute file paths.
```bash
make[1]: *** No rule to make target `test_run_dir/examples.AccumBlackBox_PeekPokeTest_Verilator345491158/AccumBlackBox.v', needed by `/Users/john/chisel-testers/test_run_dir/examples.AccumBlackBox_PeekPokeTest_Verilator345491158/VAccumBlackBoxWrapper.h'.  Stop.
```
since the path `test_run_dir/examples.AccumBlackBox_PeekPokeTest_Verilator345491158/AccumBlackBox.v` does not exist inside `test_run_dir`.
We should either:
 - strip the targetDir prefix,
 - prepend a `../` to the path,
 - use absolute paths
I decided to go with the latter since this makes the least assumptions about the actual downstream processing and we already use absolute paths in other parts of this code.

* Minor cleanup.
 - Anonymize make failure comment.
 - Use common map syntax.