Tests may invoke diff tool too many times

[ForNeVeR]

Currently, ApprovalTests.NET will call the diff tool automatically on any test failure, if the test failure has been caused by changes in the test JSON data. This is sometimes inconvenient, as with bug changes in the infrastructure it may cause huge number of diff tool invocation to start all at once. For example, see this comment.

I think we should somehow tune or disable this ApprovalTests.NET behavior.

[ygra]

DiffEngine seems to have not too many options controlling this. The best I found was to place the diff in the clipboard if the test fails.