Test: Allow to edit the left hand side of a diff editor if possible

[bpasero]

Refs: #4180

• anyOS @eamodio
• anyOS @alexandrudima

Complexity: 2

• turn auto save off
• bring up a diff editor with editable left hand side (e.g. compare 2 files from explorer, or Search & Replace preview from search viewlet)
• verify you can make changes on the left and right hand side
• verify that you can save both sides e.g. via CtrlCmd+S
• verify you cannot type into a readonly diff editor (e.g. from SCM view)
• file issues if you get confused at one point about that experience

Note: it is currently "as designed" that the left hand editor opens as a separate tab once you start editing it because the left hand editor itself does not participate in the dirty editor experience (e.g. you are not getting asked to save 2 files when you close the diff editor and have made changes to both sides).

[eamodio]

Looks good -- tested on Windows

[moshfeu]

It's open the left file in a new editor tab. I'm not sure that this is the optimal UX.

[bpasero]

As designed, otherwise we would need to show a combined save dialog when you close the editor and both sides are dirty. This solution is a good balance to keep as much of the expected behaviour as possible.