Use lexical-binding in apropos.el and add tests

Use lexical-binding in apropos.el and add tests

* lisp/apropos.el: Use lexical-binding and remove redundant
:group args.
(apropos-words-to-regexp, apropos): Tweak docstrings.
(apropos-value-internal): Replace '(if x (progn y))' with
'(when x y)'.
(apropos-format-plist): Add docstring and replace '(if x (progn y))'
with '(when x y)'.

* test/lisp/apropos-tests.el: New file with tests for apropos.el.