tests/topotests: Use copied tests in Docker

tests/topotests: Use copied tests in Docker

If we mount the tests into the container from the host, we also
mount any `*.pyc` files with them, which will lead to issues
as the mount is done read-only to avoid any changes to the host.

Since the tests are now integrated and we already create a writeable
copy of the FRR tree, just use the tests from the FRR tree to avoid
this issue.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>