Fix issue where case was mistakenly making absolute paths relative.

[ntouran]

Description

Closes #913

Though should have a test.

---

Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify that the new/changed code works.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes (location doc/release/0.X.rst) are up-to-date with any bug fixes or new features.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in setup.py.

[john-science]

@ntouran Could you add this test to the end of armi/cases/tests/test_cases.py?

    def test_copyInterfaceInputs_absPath(self):
        testSetting = "shuffleLogic"
        cs = settings.Settings(ARMI_RUN_PATH)
        shuffleFile = cs[testSetting]
        absFile = os.path.dirname(os.path.abspath(ARMI_RUN_PATH))
        absFile = str(os.path.join(absFile, os.path.basename(shuffleFile)))
        cs = cs.modified(newSettings={testSetting: absFile})

        with directoryChangers.TemporaryDirectoryChanger() as newDir:
            newSettings = cases.case.copyInterfaceInputs(
                cs, destination=newDir.destination
            )
            self.assertEqual(str(newSettings[testSetting]), absFile)

I'd do it myself, but this branch is in your fork repo.

Thanks!