Mark some slow tests as... slow

So they get spread out through the run

t/spectest.data

@@ -794,7 +794,7 @@ S16-io/tmpdir.t
 S16-io/words.t
 S16-unfiled/rebindstdhandles.t
 S17-channel/basic.t
-S17-channel/stress.t     # stress
+S17-channel/stress.t     # stress slow
 S17-lowlevel/lock.t      # slow
 S17-lowlevel/semaphore.t
 S17-lowlevel/thread.t
@@ -803,9 +803,9 @@ S17-procasync/basic.t    # moar
 S17-procasync/encoding.t # moar
 S17-procasync/print.t    # moar
 S17-procasync/kill.t     # moar stress slow
-S17-procasync/no-runaway-file-limit.t # moar
-S17-procasync/many-processes-no-close-stdin.t # moar
-S17-procasync/stress.t   # moar stress
+S17-procasync/no-runaway-file-limit.t # moar slow
+S17-procasync/many-processes-no-close-stdin.t # moar slow
+S17-procasync/stress.t   # moar stress slow
 S17-promise/allof.t      # slow
 S17-promise/at.t
 S17-promise/anyof.t