test: wait for keyring in plan submission tests

In #23977 we merged a change to how the keyring was stored. Because keyring
initialization takes slightly longer now, this uncovered existing timing bugs in
some of our tests where tests that require the keyring (ex. plan applier tests)
were waiting for the leader but not the keyring initialization. Fix another
example we've seen causing test flakes.

nomad/worker_test.go

@@ -449,7 +449,7 @@ func TestWorker_SubmitPlan(t *testing.T) {
 		c.EnabledSchedulers = []string{structs.JobTypeService}
 	})
 	defer cleanupS1()
-	testutil.WaitForLeader(t, s1.RPC)
+	testutil.WaitForKeyring(t, s1.RPC, s1.Region())
 
 	// Register node
 	node := mock.Node()