Consider subcommands when deciding to invoke Eshell command directly

Consider subcommands when deciding to invoke Eshell command directly

When an Eshell command contains an asynchronous subcommand (such as
calling an external process), it must be evaluated iteratively.  See
bug#30725.

* lisp/eshell/esh-cmd.el (eshell-invoke-command): Move most of the
logic from here...
(eshell--invoke-command-directly): ... to here. Also add checks for
subcommands.

* test/lisp/eshell/eshell-tests.el (eshell-test--max-subprocess-time):
New variable.
(eshell-wait-for-subprocess): New function.
(eshell-command-result-p): Use 'eshell-wait-for-subprocess'.
(eshell-test/interp-cmd-external): New test (bug#30725).