Fix create_breakpoint_parse_arg_string self-test

Fix create_breakpoint_parse_arg_string self-test

The emoji patch broke the create_breakpoint_parse_arg_string self-test
when gdb is running on a suitable terminal.  The problem is that the
test case doesn't take the error prefix string into account.

This patch fixes the test by having it compare the exception message
directly, rather than relying on the result of exception_print.  I did
try a different approach, of having the test mimic exception_print,
but this one seemed cleaner to me.

Approved-By: Simon Marchi <simon.marchi@efficios.com>