During completion, convert all Eshell arguments to strings

During completion, convert all Eshell arguments to strings

Eshell was already converting some types (numbers, nil) to strings, as
well as fixing up multiple-dot forms like ".../", so this way is more
consistent and should produce fewer problems for Pcomplete functions.

* lisp/eshell/em-cmpl.el (eshell-complete-parse-arguments): Always
convert parsed arguments to strings.

* test/lisp/eshell/em-cmpl-tests.el (eshell-arguments-equal,
eshell-arguments-equal--equal-explainer): New functions.
(em-cmpl-test/parse-arguments/pipeline)
(em-cmpl-test/parse-arguments/multiple-dots)
(em-cmpl-test/parse-arguments/variable/numeric)
(em-cmpl-test/parse-arguments/variable/nil)
(em-cmpl-test/parse-arguments/variable/list)
(em-cmpl-test/parse-arguments/variable/splice): Use
'eshell-arguments-equal'.