gdb/testsuite: Fix broken regexp in gdbstub case

gdb/testsuite: Fix broken regexp in gdbstub case

When $use_gdb_stub is true then, when we start an MI target there's a
regexp to match GDB's startup pattern.  Unfortunately the pattern is
broken, and we're also missing a timeout case in the match list (which
would have helped point out that the regexp was broken).

The changes to the regexp are:

  1. Remove '${run_match}' prefix, the issued command doesn't include
  '${run_prefix}' so expecting '${run_match}' is wrong.

  2. Replaced '\\n' with '\\\\n' in order to match literal '\n' in
  GDBs output (that is, match a backslash followed by 'n', not a
  newline character).

  3. Replaced a '.' (matching any character) with '\.' to match a '.'
  and moved the '\.' into the correct place in the regexp.

  4. Replaced '\r\n' with '[\r\n]+' to match the end of a line.  This
  change isn't esential, but matches the other end of line patterns
  within this regexp.

Here's an example of the output that the regexp should match taken
from a testfile log, the first line is the command sent to GDB, and
the remaining lines are the response from GDB:

  jump *_start
  &"jump *_start\n"
  ~"Continuing at 0x10074.\n"
  ^running
  *running,thread-id="all"
  (gdb)

gdb/testsuite/ChangeLog:

	* lib/mi-support.exp (mi_run_cmd_full): Fix regexp and add a
	timeout.