Provide ability to test successful issue report submission in mock environment

Background

In trac#6292 we made the issue reporter compatible with our test server for the first time. However, we still cannot reproduce parts of the flow, namely when the server returns an error (see #278 (closed)) or when the submission succeeds.

What to change

For issue reporter:

• Mock issue report submission request so that in the test environment we can control whether it should succeed or fail.
• By default, submissions should succeed.