Extend the test suite for json.el

Extend the test suite for json.el

* lisp/json.el (json-plist-p): Clarify docstring.

* test/automated/json-tests.el (json-tests--with-temp-buffer): New
macro.
(test-json-join, test-json-alist-p)
(test-json-plist-p, test-json-advance, test-json-peek)
(test-json-pop, test-json-skip-whitespace)
(test-json-read-keyword, test-json-encode-keyword)
(test-json-read-number, test-json-encode-number)
(test-json-read-escaped-char, test-json-read-string)
(test-json-encode-string, test-json-encode-key)
(test-json-new-object, test-json-add-to-object)
(test-json-read-object, test-json-encode-list)
(test-json-read-array, test-json-encode-array)
(test-json-read, test-json-read-from-string)
(test-json-encode): New tests.
(json-read-simple-alist): Merged into `test-json-read-object'.
(json-encode-string-with-special-chars): Merged into
`test-json-encode-string'.
(json-read-string-with-special-chars): Split into
`test-json-encode-string' and `test-json-read-from-string'.