After commit 9b5b0447, declaration of Groovy-based inspections moved
into gradle-groovy.xml. However, this makes transitive-optional
dependency between plugin.xml, gradle-java.xml & gradle-groovy.xml
which doesn't work (see IDEA-209769) and inspections aren't loaded.
This commit breaks this transitivity by declaring dependency
to gradle-groovy.xml directly in plugin.xml.
This also fixes most tests from GradleInspectionTest group.