Enhance syntax-tests.el to test comments in parse-partial-sexp

This now tests the interface between parse-partial-sexp and the low level
comment function in syntax.c

* test/src/syntax-tests.el (syntax-comments-midpoint): New function.
(syntax-pps-comments): New macro.
(top level): Two new syntax-br-comments tests and five new syntax-pps-comments
tests.

* test/data/syntax-comments.txt (top level): Amend some test fragments and add
some more.

test/data/syntax-comments.txt

@@ -25,18 +25,19 @@
 
 /* C Comments within lists */
 59}59
-50{ /* comment */ }50
+50{ /*70 comment */71 }50
 51{ /**/ }51
-52{ // comment
-}52
+52{ //72 comment
+73}52
 53{ //
 }53
-54{ // \
+54{ //74 \
 }54
 55{/* */}55
-56{ /* \*/ }56
-57*/57
+56{ /*76 \*/ }56
+57*/77
 58}58
+60{ /*78 \\*/79}60
 
 
 /* Straight Pascal comments (not nested) */