Further improve electric quote support for Markdown (Bug#24709)

Further improve electric quote support for Markdown (Bug#24709)

Markdown sets both 'comment-start' and 'comment-use-syntax' to non-nil
values.  Therefore 'electric-quote-mode' recognized it as a
programming mode.  Fix this by first checking whether the current
major mode is derived from 'text-mode'.

* lisp/electric.el (electric-quote-post-self-insert-function): Treat
'text-mode' as stronger signal than comment syntax.

* test/lisp/electric-tests.el (electric-quote-markdown-in-text)
(electric-quote-markdown-in-code): Adapt unit tests.