Fix test breakages caused by removal of gdb_py_test_multiple.
Tom de Vries detected that some python tests were broken as
they were still using gdb_py_test_multiple that was replaced
by gdb_test_multiline. Repair these tests by using the new function.
gdb/testsuite/ChangeLog
2020-06-30 Philippe Waroquiers <philippe.waroquiers@skynet.be>