Add tests for delete session command.

This is just a single sunshine scenario test because that's all the
command supports right now. Issues #2 and #3 highlight some of the
problems with this command, and when those issues are tackled, tests
should be introduced along with the fixes.

src/test/java/org/mobarena/stats/command/DeleteSessionStatsCommandTest.java

@@ -0,0 +1,50 @@
+package org.mobarena.stats.command;
+
+import com.garbagemule.MobArena.Messenger;
+import com.garbagemule.MobArena.framework.ArenaMaster;
+import org.bukkit.command.CommandSender;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mobarena.stats.MobArenaStats;
+import org.mobarena.stats.store.StatsStore;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import java.util.UUID;
+
+import static org.hamcrest.CoreMatchers.*;
+import static org.hamcrest.MatcherAssert.*;
+import static org.mockito.Mockito.*;
+
+@ExtendWith(MockitoExtension.class)
+class DeleteSessionStatsCommandTest {
+
+    MobArenaStats plugin;
+    DeleteSessionStatsCommand subject;
+
+    @BeforeEach
+    void setup() {
+        plugin = mock(MobArenaStats.class);
+        subject = new DeleteSessionStatsCommand(plugin);
+    }
+
+    @Test
+    void success() {
+        String id = "cafebabe-ea75-dead-beef-deadcafebabe";
+        ArenaMaster am = mock(ArenaMaster.class);
+        CommandSender sender = mock(CommandSender.class);
+        String[] args = {id};
+        Messenger messenger = mock(Messenger.class);
+        StatsStore store = mock(StatsStore.class);
+        when(am.getGlobalMessenger()).thenReturn(messenger);
+        when(plugin.getStatsStore()).thenReturn(store);
+        when(plugin.getAsyncExecutor()).thenReturn(Runnable::run);
+
+        boolean result = subject.execute(am, sender, args);
+
+        assertThat(result, equalTo(true));
+        verify(store).delete(UUID.fromString(id));
+        verify(messenger).tell(eq(sender), contains("deleted"));
+    }
+
+}