Add MockEngine content test

ktor-client/ktor-client-mock/test/io/ktor/client/tests/mock/MockEngineTests.kt

@@ -0,0 +1,36 @@
+package io.ktor.client.tests.mock
+
+import io.ktor.client.*
+import io.ktor.client.call.*
+import io.ktor.client.engine.mock.*
+import io.ktor.client.request.*
+import io.ktor.http.*
+import kotlinx.coroutines.*
+import kotlinx.coroutines.io.*
+import kotlin.test.*
+
+class MockEngineTests {
+
+    @Test
+    fun testClientMock() = runBlocking {
+        val mockEngine = MockEngine { call ->
+            if (url.encodedPath == "/") MockHttpResponse(
+                call,
+                HttpStatusCode.OK,
+                ByteReadChannel(byteArrayOf(1, 2, 3)),
+                headersOf("X-MyHeader", "MyValue")
+            ) else MockHttpResponse(
+                call, HttpStatusCode.NotFound, ByteReadChannel("Not Found ${url.encodedPath}")
+            )
+        }
+
+        val client = HttpClient(mockEngine) {
+            expectSuccess = false
+        }
+
+        assertEquals(byteArrayOf(1, 2, 3).toList(), client.get<ByteArray>("/").toList())
+        assertEquals("MyValue", client.call("/").response.headers["X-MyHeader"])
+        assertEquals("Not Found other/path", client.get<String>("/other/path"))
+    }
+
+}