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>

	* gdb.python/py-breakpoint.exp: use gdb_test_multiline instead
	of gdb_py_test_multiple.
	* gdb.python/py-cmd.exp: Likewise.
	* gdb.python/py-events.exp: Likewise.
	* gdb.python/py-function.exp: Likewise.
	* gdb.python/py-inferior.exp: Likewise.
	* gdb.python/py-infthread.exp: Likewise.
	* gdb.python/py-linetable.exp: Likewise.
	* gdb.python/py-parameter.exp: Likewise.
	* gdb.python/py-value.exp: Likewise.