Redmine#2778: acceptance test

see https://cfengine.com/dev/issues/2778

Redmine 2778 with 3.5.x: "insert_lines promise uses the same select_line_matching anchor ..."

Although the "insert_lines" seems functionally to work, nevertheless it produces some alarming, misleading and highly undesirable messages to the user.

Several of the existing acceptance tests (in "10_files/09_insert_lines") also exhibit this same undesirable behaviour.  (Such a test, e.g. "009.cf", functionally passes;  but run it manually and look at the messages.)

This test is designed to trap this behaviour and fail if the messages occur.  It seems to pass on branch "master" but fail on branch "3.5.x".

I request that this be added not only to "master" but additionally to "3.5.x".