Since the code is identical when just checking if a utility is present on
the system or not, we can factorize it.
Signed-off-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
* tests/test-lib.sh: Flag tests with missing pre-requirements as
SKIPped instead of PASSed. Do not use GNU diff extensions, but
still require a diff that understands "-u".
* tests: Do not unnecessarily require GNU diff in a lot of tests.
Make the sed utility optional.
