Fix precompiled unit tests needing their "testdata" contents

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

hack/make/test-unit

@@ -56,7 +56,7 @@ go_run_test_dir() {
 		echo
 		echo '+ go test' $TESTFLAGS "github.com/dotcloud/docker${dir#.}"
 		precompiled="$DEST/precompiled/$dir.test"
-		if ! "$precompiled"; then
+		if ! ( cd "$dir" && "$precompiled" ); then
 			TESTS_FAILED+=("$dir")
 			echo
 			echo "${RED}Tests failed: $dir${TEXTRESET}"