[interpreter] Update Makefile to ignore output of cd

The interpreter Makefile captures the output of cding into the unittest
directory and listing the unittest files it contains. Under some circumstances,
the `cd` command will print the new current directory, and that output was
incorrectly being interpreted as the path to a unittest, causing the build to
fail.

Fix this problem by redirecting the output of cd, if any, to /dev/null.

interpreter/Makefile

@@ -47,7 +47,7 @@ $(JSLIB):
 # Unit tests
 
 UNITTESTDIR =	unittest
-UNITTESTFILES =	$(shell cd $(UNITTESTDIR); ls *.ml)
+UNITTESTFILES =	$(shell cd $(UNITTESTDIR) > /dev/null; ls *.ml)
 UNITTESTS =	$(UNITTESTFILES:%.ml=%)
 
 .PHONY: unittest