add target for *-strictness tests (#662)

Author: int-e

containers-tests/tests/Makefile

@@ -10,6 +10,9 @@ all:
 %-properties: %-properties.hs force
 	ghc -I../include -O2 -DTESTING $< -i.. -o $@ -outputdir tmp/$@
 
+%-strictness: %-strictness.hs force
+	ghc -I../include -O2 -DTESTING $< -i.. -o $@ -outputdir tmp/$@
+
 %-strict-properties: %-properties.hs force
 	ghc -I../include -O2 -DTESTING -DSTRICT $< -o $@ -i.. -outputdir tmp/$@