diff: Tweak recursion tests

The -r flag is not required to compare two directories; it is only
required to compare them recursively, i.e. descend into their common
subdirectories. Adjust tests that use -r needlessly, and adjust the
dirloop test to verify that these two cases remain distinct.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D55262