indent(1): add regression test cases

These examples show expected behavior of indent(1). They are meant to be used
together with a regression test mechanism, either Kyua, a Makefile or perhaps
something else. The mechanism should in essence do this:

indent -P${test}.pro < ${test}.0 > ${test}.0.run

and compare ${test}.0.stdout to ${test}.0.run. If the files differ or the exit
status isn't 0, the test failed.

• ${test}.pro is an indent(1) profile: a list of options passed through a file. The program doesn't complain if the file doesn't exist.
• ${test}.0 is a C source file which acts as input for indent(1). It doesn't have to have any particular formatting, since it's the output that matters.
• ${test}.0.stdout contains expected output. It doesn't have to be formatted in Kernel Normal Form as the point of the tests is to check for regressions in the program and not to check that it always produces KNF.

Reviewed by: ngie
Approved by: pfg (mentor)
Differential Revision: https://reviews.freebsd.org/D9007