fix test flakiness on MjpegScreenshotTest

I realized a bit too late that the tests I added were flaky when  re-running the entire test suite:
` ./gradlew testServerDebugUnitTest   --rerun-tasks`

One of the tests would randomly (roughly 50% of the runs) fail with:
```
MjpegScreenshotTest > shouldNotBlockOnUnInitializedClient FAILED
    java.lang.AssertionError: expected:<200> but was:<-1>
        at org.junit.Assert.fail(Assert.java:89)
        at org.junit.Assert.failNotEquals(Assert.java:835)
        at org.junit.Assert.assertEquals(Assert.java:647)
        at org.junit.Assert.assertEquals(Assert.java:633)
        at io.appium.uiautomator2.server.mjpeg.MjpegScreenshotTest.shouldNotBlockOnUnInitializedClient(MjpegScreenshotTest.java:101)
```

After trying to understand what this meant: I realized I was not sending proper HTTP response, just some raw data on the socket.
It's even worse than that as we send it as streaming: the client doesn't really get an ending response.

There is no need to test that we can read a streaming response: just tweaking the mock data to send the proper header was enough to make the test reliable

app/src/test/java/io/appium/uiautomator2/server/MjpegScreenshotTest.java

@@ -32,9 +32,16 @@ public void setUp() throws Exception {
     // Create a MJPEG server with a mocked getScreenshot method
     MjpegScreenshotStream mockScreenshotStreamSpy =
         spy(new MjpegScreenshotStream(Collections.emptyList()));
-    byte[] mockScreenshotData = "screenshot data".getBytes(StandardCharsets.UTF_8);
+    String screenshotMockData = "screenshot data";
+    byte[] mockHTTPResponse =
+        ("HTTP/1.1 200 OK\n"
+                + "Content-Length: "
+                + screenshotMockData.length()
+                + "\n\n"
+                + screenshotMockData)
+            .getBytes(StandardCharsets.UTF_8);
     PowerMockito.stub(PowerMockito.method(MjpegScreenshotStream.class, "getScreenshot"))
-        .toReturn(mockScreenshotData);
+        .toReturn(mockHTTPResponse);
     PowerMockito.whenNew(MjpegScreenshotStream.class)
         .withAnyArguments()
         .thenReturn(mockScreenshotStreamSpy);