Disable empty shard slot migration test until test is de-flaked (#859)

We have a number of test failures in the empty shard migration which
seem to be related to race conditions in the failover, but could be more
pervasive. For now disable the tests to prevent so many false negative
test failures.

Signed-off-by: Madelyn Olson <madelyneolson@gmail.com>

tests/unit/cluster/slot-migration.tcl

@@ -216,6 +216,9 @@ proc create_empty_shard {p r} {
     wait_for_role $p master
 }
 
+# Temporarily disable empty shard migration tests while we
+# work to reduce their flakiness. See https://github.com/valkey-io/valkey/issues/858.
+if {0} {
 start_cluster 3 5 {tags {external:skip cluster} overrides {cluster-allow-replica-migration no cluster-node-timeout 1000} } {
 
     set node_timeout [lindex [R 0 CONFIG GET cluster-node-timeout] 1]
@@ -292,6 +295,7 @@ start_cluster 3 5 {tags {external:skip cluster} overrides {cluster-allow-replica
         wait_for_slot_state 7 "\[609-<-$R0_id\]"
     }
 }
+}
 
 proc migrate_slot {from to slot} {
     set from_id [R $from CLUSTER MYID]