Fixesscylladb/scylladb#21159
When an exception is thrown in sstable write etc such that
storage_manager::isolate is initiated, we start a shutdown chain
for message service, gossip etc. These are synced (properly) in
storage_manager::stop, but if we somehow call gossiper::shutdown
outside the normal service::stop cycle, we can end up running the
method simultaneously, intertwined (missing the guard because of
the state change between check and set). We then end up co_awaiting
an invalid future (_failure_detector_loop_done) - a second wait.
Fixed by
a.) Remove superfluous gossiper::shutdown in cql_test_env. This was added
in 20496ed, ages ago. However, it should not be needed nowadays.
b.) Ensure _failure_detector_loop_done is always waitable. Just to be sure.
Closesscylladb/scylladb#21379
c4361037f7