Added missing TruffleBoundary and added corresponding tests.

com.oracle.truffle.r.test.native/packages/testrffi/testrffi/tests/simpleTests.R

@@ -79,4 +79,13 @@ rffi.inlined_length(expr[[1]])
 
 rffi.parseVector('1+2')
 rffi.parseVector('.*/-')
-rffi.parseVector('1+')
\ No newline at end of file
+rffi.parseVector('1+')
+
+# preserve and release object
+# using loop to trigger compilation
+for(i in seq(5000)) {
+    rffi.preserve_object()
+}
+for(i in seq(5000)) {
+    rffi.release_object()
+}
\ No newline at end of file