Don't use 'list' command in Eshell command forms

Don't use 'list' command in Eshell command forms

When executed like a command, 'list' looks for external programs named
'list' first before falling back to the Lisp function of the same
name.  This causes unexpected behavior, since the Lisp function is
what we want in these tests.

* test/lisp/eshell/esh-var-tests.el (esh-var-test/interp-cmd-indices)
(esh-var-test/quoted-interp-cmd-indices): Use 'listify' instead of
'list'.