githooks: add a githook to check that the test-plans go.mod is tidied

.githooks/README.md

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+# Git Hooks
+
+This directory contains useful Git hooks for working with go-libp2p.
+
+Install them by running
+```bash
+git config core.hooksPath .githooks
+```

.githooks/pre-commit

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+#!/bin/bash
+
+pushd ./test-plans > /dev/null
+go mod tidy
+if [[ -n $(git diff --name-only -- "go.mod" "go.sum") ]]; then
+  echo "go.mod / go.sum in test-plans not tidied"
+  errored=true
+fi
+popd > /dev/null
+
+if [ "$errored" = true ]; then
+	exit 1
+fi