build: Add the "_test" suffix to test executables
build: Add the "_test" suffix to test executables

With this change, the `futures` test will produce an executable named
`tests/unit/futures_test`.

This executable can be referenced by name, or with a qualified alias:

    ninja -C build/release tests/unit/futures_test

or

    ninja -C build/release test_unit_futures

Fixes #604

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