crimson/test: chain invoke_on_all() future instead of calling get()

crimson/test: chain invoke_on_all() future instead of calling get()

The reactors start-up code on ARM64 uses invoke_on_all() to
set a configuration option.
Replace smp::invoke_on_all().get() with future chaining. This
avoids waiting on a future from a reactor continuation (outside
of a seastar thread) that throws exception.

See: https://docs.seastar.io/master/classseastar_1_1future.html#a50bfeff0acccd2f365cce40f9954218c

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>