Migrate tests to JUnit5

[Tristan-WorkGH]

• Migrate tests to JUnit5
• Migrate MockServer from JUnit4 rule to JUnit5 extensions
• Migrate from fake-sftp-server-rule JUnit4 rule to fake-sftp-server-lambda
  Suggested in Support for junit 5 stefanbirkner/fake-sftp-server-rule#30
• Fixes some problems encountered:
  • MockServer extension don't let us to set the port, so we need to be able to override the configuration at runtime
  • the CommandLineRunner will run before the @Tests as it seem to be how springboot works and no clean solution exists
    It should be possible to run tests without starting the CommandLineRunner spring-projects/spring-boot#830 (comment)
    https://www.baeldung.com/spring-junit-prevent-runner-beans-testing-execution
    So we need to mock the runner bean, then run a manual instance of it we can control.
  • the tests have been separated by class tested for better debug and control of the execution context.

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Need powsybl/powsybl-parent#65

[sonarqubecloud]

Quality Gate failed

Failed conditions
23.1% Coverage on New Code (required ≥ 80%)

See analysis details on SonarCloud