gdb: filename completion for pipe command -- the shell command bit

This commit adds filename completion for the shell command part of
the pipe command.  This is a follow on from this commit:

  commit 036e5c0c9121d0ac691dbf408a3bdf2bf3501d0f
  Date:   Mon May 19 20:54:54 2025 +0100

      gdb: use quoted filename completion for the shell command

which fixed the completion for the 'shell' command itself.

Like with the 'shell' command, we don't offer completions of command
names pulled from $PATH, we just offer filename completion, which is
often useful for arguments being passed to commands.  Maybe in the
future we could add completion for command names too (for both 'pipe'
and the 'shell' command), but that is left for a future commit.

There's some additional testing.