Fix channels test that did not wait for child context to finish

Author: steve-s

com.oracle.truffle.r.test/src/com/oracle/truffle/r/test/channels/R/sharing4.R

@@ -35,6 +35,7 @@ if (any(R.version$engine == "FastR")) {
     .fastr.channel.send(ch1, 24)
     y <- .fastr.channel.receive(ch1)
     z <- .fastr.channel.receive(ch1)
+    .fastr.context.join(cx)
 
     .fastr.channel.close(ch1)
     print(sort(c(y, z)))