Add Reassure mobile performance testing library

.gitignore

@@ -8,6 +8,7 @@ gutenberg.zip
 coverage
 *-performance-results.json
 .phpunit.result.cache
+.reassure
 
 # Directories/files that may appear in your environment
 *.log

bin/reassure-tests.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+set -e
+
+# Move out of bin directory to Gutenberg project root
+cd ../
+
+BASELINE_BRANCH=${BASELINE_BRANCH:="trunk"}
+
+# Required for `git switch` on CI
+git fetch origin
+
+# Gather baseline perf measurements
+git switch "$BASELINE_BRANCH"
+npm --cwd ci --force
+npm run native test:perf --baseline
+
+# Gather current perf measurements & compare results
+git switch --detach -
+npm ci --force
+npm run native test:perf