Move the test infrastructure to Testcontainers

[jmini]

Currently some integration are running against a GitLab started in docker, controlled by those properties in the POM:

gitlab4j-api/pom.xml

Lines 69 to 72 in b2942f3

	<gitlab.version>12.9.2-ce.0</gitlab.version>
	<gitlab.autoremove-container>true</gitlab.autoremove-container>
	<gitlab.skip-docker-start>true</gitlab.skip-docker-start>
	<gitlab.port>8090</gitlab.port>

• The GitLab version version is old.
• Tests rely on maven starting the container using the io.fabric8:docker-maven-plugin plugin.
• It is not easy to skip the integration tests on a computer where docker is not installed.

The Testcontainers is a popular project where integration tests declare their dependencies directly in the code (next to the tests).

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It would be also handy to be able to test against multiple GitLab versions, for example the one listed in the version selector on https://docs.gitlab.com/.

For example as of today this would be:

• 15.9
• 14.10
• 13.10

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This is open for suggestions

[jmini]

I am working on an other approach for our integration tests. See: #1165