Add unit tests for Dependency, improve unit testing compiler

It was not possible to make `ScalaCompilerForUnitTesting` compile several
files in different runs, which means that it was not possible to compile
and use a macro in a test case, since macros cannot be used in the same
compilation run that defines them.

This commit allows a test case to provide multiple grouped snippets of
code that will be compiled in separate runs.

For instance :
    List(List(<snippet A>, <snippet B), List(<snippet C>))

Here, <snippet A> and <snippet B> will be compiled together, and then
<snippet C> will be compiled, and will be able to use symbols defined
in <snippet A> or <snippet B>.