Fix NarTestCompileMojo resolve dependencies SNAPSHOT artifacts

In NarTestCompileMojo after resolve dependencies in downloadAttachedNars, the
SNAPSHOT dependencies will unpack as Full version name instead of SNAPSHOT
version name (e.g. hw-1.0-20140117.030545-15-x86-Windows-gpp-shared.nar). This
cause the testCompile fail if has SNAPSHOT dependencies.

The change is a workaround to explicitly unpack the NarArtifacts. This will
unpack SNAPSHOT artifacts with expected version name.