tests: add CLI dummy command-exec tool

tests: add CLI dummy command-exec tool

This adds some common CLI testtool code as well as a tool that has a
bunch of commands to be poked for their correct processing.

The tool doesn't work correctly from a script at stdin at this point
because the vty code will throw away all buffered when it sees EOF, so
the tail end of the input file is lost.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
(cherry picked from commit db93eec18d8f1e840b32ba2cdf8baf2510f6e1a5)