[tests] Prepare tests to accept a common code generator

  * Adapt tests to accept common code.generator.class property, which
    configures the code generator to use.  This enables the execution of
    top-level maven commands with the property `-Dcode.generator.class`, so
    that all tests execute with the same code generator.  The goal is to
    provide a mechanism that enables running all tests for a chosen backend.
Add 'test.excludes' property to surefire configuration

  * Enables the use of -Dtest.exclude="some java classes" to exclude them from
    the tests.  Usage example from the top-level POM, while choosing a code
    generator:

    mvn test\
    -Dcode.generator.class=pt.ist.fenixframework.backend.jvstm.JVSTMCodeGenerator\
    -Dtest.excludes="**/DmlSlotsTest.java,**/NestingTest.java" -pl test -amd
Merge pull request #130 from smmf/feature/run-all-tests-with-same-backend

Feature/run all tests with same backend

Enables picking a backend to run all tests, e.g.:

    mvn test\
      -Dcode.generator.class=pt.ist.fenixframework.backend.jvstm.JVSTMCodeGenerator

Also, we can select tests to exclude, e.g.:

    mvn test\
      -Dcode.generator.class=pt.ist.fenixframework.backend.jvstm.JVSTMCodeGenerator
      -Dtest.excludes="**/DmlSlotsTest.java,**/NestingTest.java" -pl test -amd