Enables frameworks to requires md5sum.

* The framework can now specified that it requires finding a md5sum in a page.
  This will be catched in integration tests both with the mock (medium) or
  real page (large).
* Fix a bug in the android-studio page where the md5sum was skipped.
Set real md5sum from fake android tarball
Add a context manager for tweaking files

This context manager enables changing a file content and restore the original
content.
Add a medium only test with corrupted md5sum

* This test ensure the parser get the right md5sum and that if we change
  it to a non valid value, the install fails.
Enable quitting without raising ReturnMainLoop

* Quitting does raise a ReturnMainLoop exception that is catched in mainloop.
  However, in tests, we eventually quit not from that facility. Add an option
  for that and so, remove wrong stderr output from the tests.