I don't think you can do this with ServletTester. ServletTester creates a single Context for the servlet. You need to set up embedded jetty with at least two contexts: one for the servlet, and one for the static content.
If there was a full WebAppContext, you'd be set, but there isn't.
You could make a copy of ServletTester and add hair, or you can just read up on the API and configure the necessary contexts. Here's a code fragment to show you the basic idea, you will
not be able to compile this as-is. You will need to create a suitable context for the static content.
server = new Server();
int port = Integer.parseInt(portNumber);
if (connector == null) {
connector = createConnector(port);
}
server.addConnector(connector);
for (Webapp webapp : webapps) {
File sourceDirFile = new File(webapp.getWebappSourceDirectory());
WebAppContext wac = new WebAppContext(sourceDirFile.getCanonicalPath(), webapp.getContextPath());
WebAppClassLoader loader = new WebAppClassLoader(wac);
if (webapp.getLibDirectory() != null) {
Resource r = Resource.newResource(webapp.getLibDirectory());
loader.addJars(r);
}
if (webapp.getClasspathEntries() != null) {
for (String dir : webapp.getClasspathEntries()) {
loader.addClassPath(dir);
}
}
wac.setClassLoader(loader);
server.addHandler(wac);
}
server.start();