Enhance syntax-tests.el to test nestable comments (Lisp style)

Enhance syntax-tests.el to test nestable comments (Lisp style)

Also add some tests for braces and parse-partial-sexp amongst Lisp style
comments.

* test/src/syntax-tests.el (\;-in, \;-out): Add syntax for { and }.
(top-level): Add new tests for Lisp style comments.
(\#|-in, \#|-out): New functions.
(top-level): Add new tests for nested Lisp style comments, and mixtures of
nested comments with "ordinary" comments.

* test/src/syntax-resources/syntax-comments.txt (top-level): Add new test
fragments for #|...|#, etc.