Don't apply incomplete testing configuration.

[ericsnowcurrently]

(task from #1983)

If you cancel configuration (press escape) half-way through, don't leave the settings half-configured (build up state, write everything at the end or nothing at all).

[DonJayamanne]

Workflow:

• Select configure tests
• Select pyt test
• From the next list, hit esc key

Expected behaviour:

• No changes to settings