add ut for scala command arguments

[gzm55]

supplement ut pr #586 as @slandelle suggest here

[slandelle]

I'm not fond of widening the visibility to "protected" for the sake of testing, because I've seen some people jumping into this loop hole and extend the components (yes, even with maven mojos).

IMHO, we should have the mojo do the maven plumbing, but we could resort to package protected static methods to implement the actual logic (that's what I did for "targetOption").

[gzm55]

the getScalaCommand is not easy to test or extract to a static method, it depends on the deep logic of the mojo. what's more, the tests on core static methods cannot cover the whole execution path, we still need some way to explode the field or method to the test, such as mock, override, widing visibility or even refelction.

[slandelle]

getScalaCommand doesn't do any side effect. It's just that it relies on instance fields.

We could introduce a getScalaCommand0 package protected static method that takes all the necessary parameters and have the mojo instance method call it and pass the instance fields.