feat: add MockEventProcessor implementation for testing

- Introduced `MockEventProcessor` interface in the test module with a companion object for creating mock event processors.
- Provides a `MockEventProcessorImpl` class for custom event handling logic via coroutines flow.

Signed-off-by: Forte <ForteScarlet@163.com>

Author: ForteScarlet

simbot-test/README.md

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+# simbot-test
+
+> [!warning]
+> 测试模块施工中、实验中。

simbot-test/src/commonMain/kotlin/love/forte/simbot/test/event/MockEventProcessor.kt

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+/*
+ *     Copyright (c) 2025. ForteScarlet.
+ *
+ *     Project    https://github.com/simple-robot/simpler-robot
+ *     Email      ForteScarlet@163.com
+ *
+ *     This file is part of the Simple Robot Library (Alias: simple-robot, simbot, etc.).
+ *
+ *     This program is free software: you can redistribute it and/or modify
+ *     it under the terms of the GNU Lesser General Public License as published by
+ *     the Free Software Foundation, either version 3 of the License, or
+ *     (at your option) any later version.
+ *
+ *     This program is distributed in the hope that it will be useful,
+ *     but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *     Lesser GNU General Public License for more details.
+ *
+ *     You should have received a copy of the Lesser GNU General Public License
+ *     along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ *
+ */
+
+package love.forte.simbot.test.event
+
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.flow
+import love.forte.simbot.event.Event
+import love.forte.simbot.event.EventProcessor
+import love.forte.simbot.event.EventResult
+import kotlin.jvm.JvmStatic
+
+/**
+ * Mock [EventProcessor].
+ * @since 4.13.0
+ * @author ForteScarlet
+ */
+public interface MockEventProcessor : EventProcessor {
+    public companion object {
+        public inline operator fun invoke(crossinline block: suspend (Event) -> EventResult): MockEventProcessor {
+            return create { event -> flow { block(event) } }
+        }
+
+        @JvmStatic
+        public fun create(block: (Event) -> Flow<EventResult>): MockEventProcessor {
+            return MockEventProcessorImpl(block)
+        }
+    }
+}
+
+
+private class MockEventProcessorImpl(private val processor: (Event) -> Flow<EventResult>) : MockEventProcessor {
+    override fun push(event: Event): Flow<EventResult> = processor(event)
+}