Add unit tests and documentation for Eshell predicates/modifiers

Add unit tests and documentation for Eshell predicates/modifiers

* lisp/eshell/esh-cmd.el (eshell-eval-argument): New function.
* lisp/eshell/esh-util.el (eshell-file-attributes): Pass original
value of FILE to 'file-attributes'.
* lisp/eshell/em-pred.el (eshell-predicate-alist): Change socket char
to '=', since 's' conflicts with setuid.
(eshell-modifier-alist): Fix 'E' (eval) modifier by using
'eshell-eval-argument'.  Also improve performance of 'O' (reversed
sort) modifier.
(eshell-modifier-help-string): Fix documentation of global
substitution modifier.
(eshell-pred-substitute): Fix infinite loop in some global
substitutions.
(eshell-join-members): Fix joining with implicit " " delimiter.
(Bug#54470)

* test/lisp/eshell/em-pred-tests.el: New file.

* doc/misc/eshell.texi (Argument Predication): New section.