add --line-buffered

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)

hack/make/test-integration

@@ -12,5 +12,5 @@ bundle_test_integration() {
 # this "grep" hides some really irritating warnings that "go test -coverpkg"
 # spews when it is given packages that aren't used
 bundle_test_integration 2>&1 \
-	| grep -v '^warning: no packages being tested depend on ' \
+	| grep --line-buffered -v '^warning: no packages being tested depend on ' \
 	| tee $DEST/test.log