Update make to generate `testdata/scratch` dir

When running the gitaly tests for the first time to see an error message
to run `make prepare-tests` but then after running the tests again you
start seeing the following error message:

```
Error:  Received unexpected error:
listen unix testdata/scratch/gitaly-8674665223082153551.sock: bind: no
such file or directory
```

This is because the directory `testdata/scratch` is not created.

Makefile

@@ -102,11 +102,15 @@ clean-build:
 
 .PHONY:	prepare-tests
 prepare-tests:	testdata/data/group/test.git $(EXE_ALL)
+prepare-tests:	testdata/scratch
 
 testdata/data/group/test.git:
 	$(call message,$@)
 	git clone --quiet --bare https://gitlab.com/gitlab-org/gitlab-test.git $@
 
+testdata/scratch:
+	mkdir -p testdata/scratch
+
 .PHONY: verify
 verify: lint vet detect-context check-formatting staticcheck