Move Eshell variable interpolation tests to their own file

Move Eshell variable interpolation tests to their own file

* test/lisp/eshell/eshell-tests.el (eshell-test/interp-cmd)
(eshell-test/interp-lisp, eshell-test/interp-temp-cmd)
(eshell-test/interp-concat, eshell-test/interp-concat-lisp)
(eshell-test/interp-concat2, eshell-test/interp-concat-lisp2)
(eshell-test/interp-cmd-external)
(eshell-test/interp-cmd-external-concat, eshell-test/window-height)
(eshell-test/window-width, eshell-test/last-result-var)
(eshell-test/last-result-var2, eshell-test/last-arg-var):
Move from here...

* test/lisp/eshell/esh-var-test.el (esh-var-test/interp-lisp)
(esh-var-test/interp-cmd, esh-var-test/interp-cmd-external)
(esh-var-test/interp-temp-cmd, esh-var-test/interp-concat-lisp)
(esh-var-test/interp-concat-lisp2, esh-var-test/interp-concat-cmd)
(esh-var-test/interp-concat-cmd2)
(esh-var-test/interp-concat-cmd-external, esh-var-test/window-height)
(esh-var-test/window-width, esh-var-test/last-result-var)
(esh-var-test/last-result-var2, esh-var-test/last-arg-var):
... to here.