tools: add download testing

This script will take an environment variable (NODE_VERSION) and
download all of the files from `http://nodejs.org/dist/v<NODE_VERSION>/`
It will then loop through the `SHASUMS256.txt` file to check if the
SHA256SUMS match up. Initial issue here:
#513

jenkins/download-test.sh

@@ -0,0 +1,66 @@
+#!/bin/bash
+# Pass `NODE_VERSION` as an environment variable. Can be an exact or vague version.
+# NODE_VERSION=v8 ./download-test.sh --tap ./test.tap
+
+while [ $# -gt 1 ]; do
+
+  case "$1" in
+      -t|--tap)
+      tapFile="$2"
+      shift # Past option argument.
+      ;;
+      *) # unknown option
+      echo "Ignoring unknown option: $1"
+      ;;
+  esac
+  shift # Past option.
+done
+
+if [ "$DOWNLOAD_LOCATION" == "Nightly" ]; then
+  downloadDir="nightly"
+else
+  downloadDir="release"
+fi
+
+if [ -z "$NODE_VERSION" ]; then
+  echo "NODE_VERSION is undefined! Please declare NODE_VERSION"
+  exit 1
+fi
+
+# If the provided NODE_VERSION is inexact, e.g. `v8`, get the most recent matching version.
+LINK=`rsync rsync://unencrypted.nodejs.org/nodejs/$downloadDir/ | grep $NODE_VERSION | sort -t. -k 1,1n -k 2,2n -k 3,3n | tail -1 | awk '{print $5}'`
+
+# Remove old files
+rm -rf v*
+[ "$tapFile" ] && rm -rf "$tapFile" && echo "TAP version 13" > $tapFile
+
+# Scrape the download server
+rsync -av rsync://unencrypted.nodejs.org/nodejs/"$downloadDir"/"$LINK" .
+
+testNumber=0
+cat "$LINK"/SHASUMS256.txt | {
+  while read line; do
+   let testNumber++ # Increment at the beginning
+   sha=$(echo "${line}" | awk '{print $1}')
+   file=$(echo "${line}" | awk '{print $2}')
+   echo "Checking shasum for $file"
+   calculatedSHA=$(shasum -a 256 "$LINK/$file" | awk '{print $1}')
+   if [ "$calculatedSHA" != "$sha" ]; then
+     echo "Error - SHASUMS256 does not match for $file"
+     echo "Expected - $sha"
+     echo "Found - $calculatedSHA"
+     if [ "$tapFile" ]; then
+       echo "not ok $testNumber SHASUMS256 does not match for $file
+         ---
+           Expected - $sha
+           Found - $remoteSHA
+         ...
+       " >> "$tapFile"
+     fi
+   else
+     echo "Pass - SHASUMS256 is correct for $file"
+     [ "$tapFile" ] && echo "ok $testNumber SHASUMS256 for $file is correct" >> "$tapFile"
+   fi
+  done
+[ "$tapFile" ] && echo "1..$testNumber" >> "$tapFile"
+}