MDL-37354 clean return value for each externallib PHPUnit tests

Conflicts:
	enrol/tests/externallib_test.php
	group/tests/externallib_test.php
	lib/external/tests/externallib_test.php
	message/tests/externallib_test.php
	mod/assign/tests/externallib_test.php
	notes/tests/externallib_test.php
	webservice/tests/externallib_test.php