Use pytest-flaky to retry fragile functional tests.

[ntoll]

Sometimes, on CI, our functional tests fail with errors like this:

sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) unable to open database file

Such random, impossible to reproduce and frustrating errors are caused by a problematic state in the CI server, rather than with our tests. For instance, the error quoted above is due to disk read/write problems on the server.

I suggest we use pytest-flaky as a way to retry such flaky tests.

[kushaldas]

I saw the same error today:

sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) unable to open database file
(Background on this error at: http://sqlalche.me/e/e3q8)

[kushaldas]

Now I am seeing this error in all random tests in both master and while working on https://github.com/freedomofpress/securedrop-client/tree/login_via_clicking_icon_functional_test I think I should just add flaky to all tests under functional