[jvm-packages] allow partial evaluation of dataframe before prediction (#4407)

* allow partial evaluation of dataframe before prediction

* resume spark test

* comments

* Run unit tests after building JVM packages

tests/ci_build/build_jvm_packages.sh

@@ -9,7 +9,7 @@ set -x
 rm -rf build/
 cd jvm-packages
 
-mvn --no-transfer-progress package -DskipTests
+mvn --no-transfer-progress package
 
 set +x
 set +e