test: fix unit test flakiness on MjpegScreenshotTest

[vbarbaresi]

I realized a bit too late that the tests I added in #676 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)

I realized I was not sending a proper HTTP response, just some raw data on the socket (the HTTP header is added in getScreenshot() method, which was mocked)
As we send this data as streaming: the HTTP client doesn't really know how to interpret this data.
It was surprising that it worked in most of the test runs.

There is no need to test that we can really read this streaming response: just tweaking the mock data to send the proper header was enough to make the test reliable and see the response as 200.

• Skipped those tests in the CI using an environment variable to avoid flakiness issues
  For the record: adding a Thread.sleep(1000) after starting the serverThread made the test pass in the CI, but it's not going to be 100% reliable. Easier to skip if for now

[mykola-mokhnach]

the PR prefix should rather be test:. It should not bump the module version as it does not introduce any changes to the production code

[vbarbaresi]

Changed the PR prefix
Though it seems CI has failed 🤔 It was 100% reliable on my machine this time

[mykola-mokhnach]

Changed the PR prefix Though it seems CI has failed 🤔 It was 100% reliable on my machine this time

GH CI is slow, so it might (and usually does) affect the result.
You might skip the test conditionally if it is running in the CI env by setting the CI environment variable in the workflow and then checking for its presence in the test.

[vbarbaresi]

You might skip the test conditionally if it is running in the CI env by setting the CI environment variable in the workflow and then checking for its presence in the test.

Done 👍
I also updated the PR description: test is now conditionally skipped in the CI.

[github-actions]

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