build: Compile `seastar_testing` with dev. flags
build: Compile `seastar_testing` with dev. flags

We want the development flags (`Seastar_DEVELOPMENT_*`) to also be used
when compiling the Seastar testing library.

This was an accidental omission from
5cefd044ae5cc5f8d017579962779b5cb6cf3bfd.

Tests: dist (release)

Fixes #575

Signed-off-by: Jesse Haber-Kucharsky <526eadea4792054b99d5fa5dd6be9acdd76c5418@scylladb.com>
Message-Id: <864fbd4a943d6068098726920bceac3d450ce72d.1547155354.git.526eadea4792054b99d5fa5dd6be9acdd76c5418@scylladb.com>

cloudius-systems/seastar,cloudius-systems/seastar,avikivity/seastar,dreamsxin/seastar,scylladb/seastar,avikivity/seastar,dreamsxin/seastar,syuu1228/seastar,syuu1228/seastar,avikivity/seastar,syuu1228/seastar,dreamsxin/seastar,scylladb/seastar,cloudius-systems/seastar,scylladb/seastar