indent: Simplify test script

Much of the test script was concerned with working around RCS tags and
dealing with cases which don't apply to indent.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D51795

(cherry picked from commit 0ecc679fd3da0720d6ba331ebcceca8e0760fa22)