gdb/testsuite: use gdb_test in gdb.gdb/python-helper.exp

gdb/testsuite: use gdb_test in gdb.gdb/python-helper.exp

If some command in there gives the wrong answer, we currently have to
wait for a timeout for the test to continue.  For instance, I currently
see:

    print *val->type
    $1 = Python Exception <class 'gdb.error'>: Cannot take address of method length.

    (outer-gdb) FAIL: gdb.gdb/python-helper.exp: pretty print type (timeout)

We can avoid this and modernize the test at the same time by using the
-prompt option of gdb_test.

gdb_test_no_output currently accepts a -prompt_re option (the variable
name passed to parse_args defines the option name), but I think
it's a typo.  It's supposed to be -prompt, like gdb_test.  I can't find
anything using -prompt_re using grep.  Change it to just "prompt".

Change-Id: Icc0a9a0ef482e62460c708bccdd544c11d711eca