only install packages during testing if there are tests requiring them

...ruffle.r.test/src/com/oracle/truffle/r/test/TestBase.java

...com/oracle/truffle/r/test/generate/TestOutputManager.java

...t/src/com/oracle/truffle/r/test/rffi/TestRFFIPackage.java

...rc/com/oracle/truffle/r/test/rpackages/TestRPackages.java

...cle/truffle/r/test/rpackages/TestRecommendedPackages.java

...om/oracle/truffle/r/test/rpackages/TestS4TestPackage.java

...m/oracle/truffle/r/test/rpackages/TestVanillaPackage.java

mx.fastr/mx_fastr.py