Add integration testing

[pyjarrett]

Integration tests would help verify behavior and provide examples of what Septum can do.

• Provide a small interior project with source code for searching.
• Provide a command file, which gets executed as if run by user input.
• Given the command file, verify Septum's output against that an expected output file.

This should all be done without any external programs to simplify portability to multiple platforms.

Possible implementation:

• Create a separate septum-integration-tests.gpr file which uses GNAT.OS_Lib and/or GNAT.Expect for building and running septum. See AdaCore gem 54 for examples