tests: actually have TestSessionTTLRenew sleep during execution (#5669)

Due to an unintended order of operations issue with integer division
TestSessionTTLRenew was sleeping for 0s every time.

Also add explicit failures for when the various session renewal returns
nil unexpectedly.

agent/session_endpoint_test.go

@@ -397,7 +397,11 @@ func TestSessionTTLRenew(t *testing.T) {
 	}
 
 	// Sleep to consume some time before renew
-	time.Sleep(ttl * (structs.SessionTTLMultiplier / 3))
+	sleepFor := ttl * structs.SessionTTLMultiplier / 3
+	if sleepFor <= 0 {
+		t.Fatalf("timing tests need to sleep")
+	}
+	time.Sleep(sleepFor)
 
 	req, _ = http.NewRequest("PUT", "/v1/session/renew/"+id, nil)
 	resp = httptest.NewRecorder()
@@ -405,6 +409,9 @@ func TestSessionTTLRenew(t *testing.T) {
 	if err != nil {
 		t.Fatalf("err: %v", err)
 	}
+	if obj == nil {
+		t.Fatalf("session '%s' expired before renewal", id)
+	}
 	respObj, ok = obj.(structs.Sessions)
 	if !ok {
 		t.Fatalf("should work")