[SUREFIRE-1876] Possibility to skip unit but not integration tests

**[Tobias Stadler](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=tstadler)** opened **[SUREFIRE-1876](https://issues.apache.org/jira/browse/SUREFIRE-1876?redirect=false)** and commented

Currently one can skip execution of integration test while still executing unit tests by using the skipIT property. I would would like to be able to skip execution of unit tests but not the integration tests.

My integration tests are located in a module of a multi module project. When running mvn verify -pl :my-it-project -am both unit and integration tests for all projects are run. Skipping unit test execution would speed things up a little bit.

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**Affects:** 3.0.0-M5

**Issue Links:**
- [SUREFIRE-1170](https://issues.apache.org/jira/browse/SUREFIRE-1170) skiptTests=true is used by maven-surefire-plugin as well as of maven-failsafe-plugin
(_**"is duplicated by"**_)

**Remote Links:**
- [GitHub Pull Request #331
](https://github.com/apache/maven-surefire/pull/331)

Start by reading the maven-surefire-plugin and maven-failsafe-plugin behavior described in the issue, including the existing skipIT and skipTests properties. Review the linked Apache JIRA issues and GitHub pull request #331 to understand prior work and the relationship between unit and integration test execution. Done means unit tests can be skipped while integration tests still run for the described multi-module Maven command.

Written by the indexing model from the issue text.