Install browsers in the buildbot container and set CHROME_BIN for npm test

.buildbot/node/Dockerfile

@@ -1,7 +1,8 @@
 FROM node:18-bullseye
 
 RUN apt-get update
-RUN apt-get install -yq --no-install-suggests --no-install-recommends xauth xvfb
+RUN apt-get install -yq --no-install-suggests --no-install-recommends \
+    xauth xvfb chromium firefox-esr
 
 WORKDIR /app
 COPY package*.json binding.gyp *.cc ./

.buildbot/node/test.sh

@@ -1,3 +1,3 @@
 #!/bin/sh
 
-npm test
+CHROME_BIN=/usr/bin/chromium npm test