Refactor the TestRebase integration test

Start of the refactoring of the `scc` library integration tests
- Add generic `open_pr()` method to `SandboxTest`. Allows to open a new test PR against the sandbox repository using the following:

 ```
self.fake_branch(branch)
self.open_pr(branch, base)
```
- Add tests for the base of rebased PR in TestRebase


/cc @knabar