testsuite: skip confirmation in 'gdb_reinitialize_dir'

testsuite: skip confirmation in 'gdb_reinitialize_dir'

Some shells automatically confirm the 'dir' command:

  (gdb) dir
  Reinitialize source path to empty? (y or n)
    [answered Y; input not from terminal]
  Source directories searched: $cdir;$cwd
  (gdb) y
  dir <...>/gdb/testsuite/gdb.base
  Undefined command: "y".  Try "help".

For example, this reprdocues in a MinGW32 environment with
'TERM=dumb'.  Skip sending 'y' if the command is already confirmed.

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