Improve tests/organization for built-in variables

Improve tests/organization for built-in variables

* lisp/eshell/em-dirs.el (eshell-inside-emacs)
(eshell-dirs-initialize): Move 'INSIDE_EMACS' from here...

* lisp/eshell/esh-var.el (eshell-inside-emacs)
(eshell-variable-aliases-alist): ... to here, and improve doc string.

* test/lisp/eshell/eshell-tests.el (eshell-test/inside-emacs-var):
Move from here...

* test/lisp/eshell/esh-var-tests.el (esh-var-test/inside-emacs-var):
... to here.
(esh-var-test/last-arg-var-indices)
(esh-var-test/last-arg-var-split-indices): New tests.

* test/lisp/eshell/em-alias-tests.el:
* test/lisp/eshell/em-dirs-tests.el:
* test/lisp/eshell-em-script-tests.el: New files.

* doc/misc/eshell.texi (Built-ins): Fix 'cd' documentation; it works
with the directory ring, not the directory stack.  Move built-in
variables documentation from here...
(Variables): ... to here, and add documentation for missing built-in
variables.