Update log file diff in post.sh

Prior to this change log file diffs in unit tests compared the reference file to the current output, rather than the other way around, so that lines in the log removed due to local changes are marked with a + in the diff output and lines added to the log are marked with a -.

This commit switches the order of comparison with the aim of making the output more intuitive. See discussion in ATEAM-644.

Closes ATEAM-644

This merge request affects 1 package:

• AtlasTest/TestTools

added 21.6 Test review-pending-level-1 labels

CI Result SUCCESS (hash 45fd1c43)

	AthGeneration
externals
cmake
make
required tests
optional tests

Full details available on this CI monitor view
AthGeneration: number of compilation errors 0, warnings 1
For experts only: Jenkins output [CI-MERGE-REQUEST 44726]

merged

mentioned in commit 93908463

added sweep:ignore label

21.6.40,AthGeneration with this MR is built and awaits cvmfs distribution: https://its.cern.ch/jira/browse/ATLINFR-3700