Add tests for varchar() and combinations of string types.

src/test/regress/sql/strings.sql

@@ -0,0 +1,23 @@
+--
+-- Test various data entry syntaxes.
+--
+
+-- SQL92 string continuation syntax
+SELECT 'first line'
+' - next line'
+	' - third line'
+	AS "Three lines to one";
+
+-- illegal string continuation syntax
+SELECT 'first line'
+' - next line' /* this comment is not allowed here */
+' - third line';
+
+--
+-- test conversions between various string types
+--
+
+SELECT text(f1) FROM CHAR_TBL;
+
+SELECT text(f1) FROM VARCHAR_TBL;
+