unicode: build maketables during testshort too

Fixes #1825

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/4535066

src/pkg/unicode/Makefile

@@ -26,6 +26,7 @@ tables:	maketables
 # Build (but do not run) maketables during testing,
 # just to make sure it still compiles.
 test: maketables
+testshort: maketables
 
 # Downloads from www.unicode.org, so not part
 # of standard test scripts.