ctdb-build: Separate test backtrace support into separate subsystem

ctdb-build: Separate test backtrace support into separate subsystem

A convention when testing members of ctdb-util is to include the .c
file so that static functions can potentially be tested.  This means
that such tests can't be linked against ctdb-util or duplicate symbols
will be encountered.

ctdb-tests-common depends on ctdb-client, which depends in turn on
ctdb-util, so this can't be used to pull in backtrace support.
Instead, make ctdb-tests-backtrace its own subsystem.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>