Following guidelines to ensure argLine values set by other plugins is preserved

Following guidelines to ensure argLine values set by other plugins is preserved. Addressing review comment from !247 (merged)

More at https://stackoverflow.com/questions/46489455/append-the-value-of-argline-param-in-maven-surefire-plugin and https://maven.apache.org/surefire/maven-surefire-plugin/test-mojo.html#argLine