Fix combat brace slayer task race condition

Apparently combat bracelet message is sent before XP drop is received,
so task updates to correct amount and then is substracted by 1. To
combat this, virtually increase amount by 1 so the xp drop will
substract to correct amount.

Closes #6091

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

runelite-client/src/test/java/net/runelite/client/plugins/slayer/SlayerPluginTest.java

@@ -429,6 +429,7 @@ public class SlayerPluginTest
 		slayerPlugin.onChatMessage(chatMessage);
 
 		assertEquals("Suqahs", slayerPlugin.getTaskName());
+		slayerPlugin.killedOne();
 		assertEquals(30, slayerPlugin.getAmount());
 	}
 }