Maven: skip also integration test when the global "skip tests" option is enabled

[NicolaIsotta]

Starting with Surefire/Failsafe 3.6.0, skipTests only skips unit tests. Set the skipITs property alongside skipTests in TestChecker when test skipping is enabled to also skip integration test execution.

For reference:

• https://maven.apache.org/surefire/maven-failsafe-plugin/integration-test-mojo.html#skipITs
• https://maven.apache.org/surefire/#3-6-0
• [SUREFIRE-823] Decouple -DskipTests from Failsafe plugin maven-surefire#3371

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