[gdb/testsuite] Don't timeout on prompt in gdb_start_cmd

We're currently running into a timeout at:
...
(gdb) start ^M
Error in expression, near `1'.^M
(gdb) UNTESTED: gdb.ada/start.exp: start failed to land inside the right \
  procedure
...
due to the fact that gdb_start_cmd doesn't handle a prompt as reaction to
the start command.

Fix this by handling the prompt.  Reduces execution time of the test-case from
1m1s to 1s.

Tested on x86_64-linux.