minor, introduce a test-id that remains constant through the test

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
hygiene, open() files for the diff tool with static flags

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'code/hygiene'

* code/hygiene:
  hygiene, open() files for the diff tool with static flags
  minor, introduce a test-id that remains constant through the test
fix, PEBKAC, finalise the file names for diff subprocess

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>