tools: add prepare-source.sh

A script that runs various go tools (mod tidy, mod vendor, and fmt for
now).

The idea is that it prepares the source to be ready for master. As such,
running it on master shouldn't modify any files. Make sure of that by
adding a test.

tools/prepare-source.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+set -eux
+
+go fmt ./...
+go mod tidy
+go mod vendor