gdb/testsuite: Use default gdb_expect timeout in runto

gdb/testsuite: Use default gdb_expect timeout in runto

runto uses a hard-coded timeout of 30s in its invocation of gdb_expect.
This is normally fine, but for very a slow system (e.g., an emulator) it
may not be enough time for GDB to reach the intended breakpoint.

gdb_expect can obtain a timeout value from user-configurable variables
when it's not given one explicitly, so use that mechanism instead since
the user will have already adjusted the timeout variable to account for
the slow system.

Approved-By: Tom Tromey <tom@tromey.com>