Development: Fix flaky PostIntegrationTest

[laadvo]

Checklist

General

• Language: I followed the guidelines for inclusive, diversity-sensitive, and appreciative language.
• I chose a title conforming to the naming conventions for pull requests.

Server

• I followed the coding and design guidelines.
• I documented the Java code using JavaDoc style.

Motivation and Context

The test PostIntegrationTest.testGetPostsForCourse_WithExerciseIdRequestParam() is flaky and sometimes fails with

org.mockito.exceptions.misusing.UnfinishedStubbingException: org.mockito.exceptions.misusing.UnfinishedStubbingException: 
Unfinished stubbing detected here:
-> at de.tum.in.www1.artemis.util.AbstractArtemisIntegrationTest.mockMailService(AbstractArtemisIntegrationTest.java:129)

E.g. thenReturn() may be missing.
Examples of correct stubbing:
    when(mock.isOk()).thenReturn(true);
    when(mock.isOk()).thenThrow(exception);

This PR fixes the test testCreateAnnouncement() which is executed before testGetPostsForCourse_WithExerciseIdRequestParam() and is causing it to be fail.

Description

In the test PostIntegrationTest.testCreateAnnouncement() mails are sent using the javaMailSender in async. The mails sometimes get sent after the javaMailSender already mock got reset. When the mock is configured again in the @BeforeEach method mockMailService() there already exists a recorded call on the mock which causes the cryptic UnfinishedStubbingException.
This happened in the @BeforeEach of testGetPostsForCourse_WithExerciseIdRequestParam() which is always run directly after testCreateAnnouncement().

The PR adds a verify statement at the end of testCreateAnnouncement(), making sure that the mails are sent before starting the next test.

Review Progress

Look at the code changes and feel free to run PostIntegrationTest multiple times using the following script (inspired by the script from #6506).

rm postIntegration.log
for i in {1..50}
do
   echo "Starting run: i=$i."
   ./gradlew cleanTest test --tests de.tum.in.www1.artemis.metis.PostIntegrationTest -x webapp | tee -a postIntegration.log
done

echo "Print failed tests:"
cat postIntegration.log | grep "testGetPostsForCourse_WithExerciseIdRequestParam() FAILED" || [[ $? == 1 ]]

Code Review

• Code Review 1
• Code Review 2

Test Coverage

unchanged

[DominikRemo]

Changes LGTM. Test succeeds locally

[Strohgelaender]

code lgtm
thanks a lot for figuring this out.

[MaximilianAnzinger]

LGTM

[dearjasmina]

code lgtm