fix: string(int) expression not allowed on Go1.15 (pingcap#1003)

Make test will fail due to Go1.15 starts to not allow "string(int)" expression.
Fix affected sources by replace such expression to legal expressions.

More info about ban string(int), see: golang/go#3939

Co-authored-by: ti-srebot <66930949+ti-srebot@users.noreply.github.com>

Author: LENSHOOD

ast/ddl.go

@@ -1077,7 +1077,7 @@ func (n *DropTableStmt) Restore(ctx *format.RestoreCtx) error {
 			ctx.WritePlain(", ")
 		}
 		if err := table.Restore(ctx); err != nil {
-			return errors.Annotate(err, "An error occurred while restore DropTableStmt.Tables "+string(index))
+			return errors.Annotatef(err, "An error occurred while restore DropTableStmt.Tables[%d]", index)
 		}
 	}
 

lexer_test.go

@@ -201,7 +201,7 @@ func (s *testLexerSuite) TestscanString(c *C) {
 		{`' \n\tTest String'`, " \n\tTest String"},
 		{`'\x\B'`, "xB"},
 		{`'\0\'\"\b\n\r\t\\'`, "\000'\"\b\n\r\t\\"},
-		{`'\Z'`, string(26)},
+		{`'\Z'`, "\x1a"},
 		{`'\%\_'`, `\%\_`},
 		{`'hello'`, "hello"},
 		{`'"hello"'`, `"hello"`},