[GR-2311] Modifications for unit tests (SVM).

...e/src/com/oracle/truffle/r/engine/interop/RContextMR.java

...e/truffle/r/engine/interop/RForeignAccessFactoryImpl.java

...r.engine/src/com/oracle/truffle/r/engine/shell/RMain.java

...me/src/com/oracle/truffle/r/runtime/context/RContext.java

...racle/truffle/r/runtime/ffi/managed/Managed_PCRERFFI.java

...test/src/com/oracle/truffle/r/test/FastRJUnitWrapper.java

mx.fastr/mx_fastr_junit.py

mx.fastr/suite.py