add test for vanilla package loading and enable in gate

.hgignore

...oracle.truffle.r.test/rpackages/justr/vanilla/DESCRIPTION

com.oracle.truffle.r.test/rpackages/justr/vanilla/NAMESPACE

...oracle.truffle.r.test/rpackages/justr/vanilla/R/vanilla.R

...fle.r.test/rpackages/justr/vanilla/man/vanilla-package.Rd

...cle.truffle.r.test/rpackages/justr/vanilla/man/vanilla.Rd

...est/src/com/oracle/truffle/r/test/ExpectedTestOutput.test

...m/oracle/truffle/r/test/rpackages/RPackagesWhiteList.test

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

mx.fastr/mx_fastr.py