Fix build-and-build-again bug in sunrpc tests.

rpcgen will error out if the file it's asked to create already exists.
Several other rules in sunrpc/Makefile take care to delete rpcgen-
generated files before creating them, but rpcgen-tests doesn't, which
can lead to spurious test failures in an incremental rebuild.

	* sunrpc/Makefile (rpcgen-tests): Delete the .out file before
	creating or re-creating it.