Improve behavior of 'electric-pair-mode' in 'cc-mode'

Improve behavior of 'electric-pair-mode' in 'cc-mode'

* lisp/progmodes/cc-mode.el (c-electric-pair-inhibit-predicate):
Inhibit insertion of paired quote in fewer cases.
* test/lisp/electric-tests.el (define-electric-pair-test):
Add 'c-mode' to list of modes to test by default (bug#50538).

This ensures that quotes are paired correctly within comments, allows for
insertion of quote pairs immediately before another quote, and allows
inserting quote pairs within a string (thus splitting the string in two).