Respect buffer-local value of tags-table-list (Bug#27772)

Respect buffer-local value of tags-table-list (Bug#27772)

* lisp/progmodes/etags.el (visit-tags-table-buffer): Save the current
buffer around the `tags-table-including' calls so as to get buffer
local variables from the right buffer later.
* test/lisp/progmodes/etags-tests.el (etags-visit-tags-table-buffer):
New test.
* test/lisp/progmodes/etags-tests.el (etags-tests--test-dir): New
constant.
(etags-bug-158, etags-bug-23164): Use it so that when running the test
interactively, setting EMACS_TEST_DIRECTORY is not needed.