[SPARK-4458][Build] Eliminate compilation of test classes in make distribution.

The make-distribution generates Spark distributions, and therefore does not require building of test classes. Maven -DskipTests only eliminates running of tests, but does not eliminate compilation of tests (see [Maven docs](http://maven.apache.org/surefire/maven-surefire-plugin/test-mojo.html#skipTests)). To prevent compilation of tests (reduces compilation time), we need to add -Dmaven.test.skip (again see [Maven docs](http://maven.apache.org/surefire/maven-surefire-plugin/test-mojo.html#skip)).