Add explicit test for various comment syntaxes.

src/test/regress/sql/comments.sql

+--
+-- Comments
+--
+
+SELECT 'trailing' AS first; -- trailing single line
+SELECT /* embedded single line */ 'embedded' AS second;
+SELECT /* both embedded and trailing single line */ 'both' AS third; -- trailing single line
+
+SELECT 'before multi-line' AS fourth;
+/* This is an example of SQL which should not execute:
+ * select 'multi-line';
+ */
+SELECT 'after multi-line' AS fifth;
+
+/* and this is the end of the file */
+

src/test/regress/sql/tests

@@ -27,6 +27,7 @@ reltime
 abstime
 tinterval
 horology
+comments
 create_function_1
 create_type
 create_table