gdb/testsuite: Make test names unique in gdb.python/py-prettyprint.exp

gdb/testsuite: Make test names unique in gdb.python/py-prettyprint.exp

This makes the test names unique in gdb.python/py-prettyprint.exp, it
also switches to use gdb_breakpoint and gdb_continue_to_breakpoint
more so that we avoid test names with the source line number in - this
is bad if the test source ever changes as the test names will then
change.

One final change is to switch from using gdb_py_test_silent_cmd to use
gdb_test_no_output, the former should be used for running python
commands and can catch any thrown exception.  However, in this case
the command being run is not a python command, its just a normal GDB
CLI command that produces no output, so lets use the appropriate
wrapper function.

gdb/testsuite/ChangeLog:

	* gdb.python/py-prettyprint.exp: Use gdb_breakpoint and
	gdb_continue_to_breakpoint more throughout this test.
	(run_lang_tests) Supply unique test names, and use
	gdb_test_no_output.