Simplify test_verify.py to just do a simple backup and verify on a single file in each test. Modify tests to correctly use --compare-data option and to add tests for verify when the source files have the atime/mtime manipulated.
* Merge in lp:~hooloovoo/duplicity/test-verify-improvements
  - Fix up test_verify, which was a bit of a mess:
  - Simplify test_verify.py to just do a simple backup and verify on a
    single file in each test.
  - Modify tests to correctly use --compare-data option.
  - Add tests for when the source files have atime/mtime manipulated.
* Fix duplicity verify to ignore the file system when globals.compare_data is
  False.  This means that verify only validates the viability of the backup
  itself unless --compare-data is specified.
* Reenable test_verify_changed_source_file test