[interpreter] Update Makefile to ignore output of cd (#1683)

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.

Fixes #1681.

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
@@ -62,7 +62,7 @@ unittest/%:
 # Test suite
 
 TESTDIR =	../test/core
-TESTFILES =	$(shell cd $(TESTDIR); ls *.wast; ls [a-z]*/*.wast)
+TESTFILES =	$(shell cd $(TESTDIR) > /dev/null; ls *.wast; ls [a-z]*/*.wast)
 TESTS =		$(TESTFILES:%.wast=%)
 
 .PHONY: test partest quiettest