Disable linting by default for test windows

[redmunds]

Now that we have project-specific preferences, this should default to off so linting doesn't run for every file opened in a test window during unit tests.

Or maybe this can be turned off for test windows, and on for normal windows?

[peterflynn]

Yeah, it seems like we shouldn't let our unit testing needs override what we think is the best default for our users...

[dangoor]

This is easy to do once #7163 lands. TestPreferenceImpl can just set linting.enabled to false.