Regression Script Refactoring

# Changes

### Renaming
* Renamed test_case variable to more descriptive test_config
* Renamed Test_Runner to TestRunner
* Renamed DatabaseDiff to TskDbDiff

### TestData
* Added helper methods to TestData to simplify access to database paths [AUT-1063]

### TestConfiguration
* Removed dependency of TestConfiguration on TestData to print errors
* Moved additional configuration logic from run_test methods to TestConfiguration class
* Consolidated redundant attributes in TestConfiguration

### TestRunner
* Created a function to generate the TestData objects instead of continually clearing and changing the attributes in a single TestData object [AUT-1034]

### TskDbDiff
* Abstracted similar methods in TskDbDiff
* Eliminated reuse of make_path function