tests: make a plan for man page checks

Split the man page check routine into two routines; one to get the list
of sub-commands for a command, and another to verify a man page exists
for each sub-command.  Use the list of sub-commands to set up the
Test::More plan before running the tests.

Setting the plan before running the tests allows the the man page tests
to run on systems which ship older versions the Test::More module.

Change-Id: I1ed6fb87989e1deff4696562f3b917140592ed17
Reviewed-on: http://gerrit.openafs.org/9835
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>