This looks great! I'm excited to be able to run some tests here to verify things look correct across different versions of Prerender and different versions of Chrome. I'll add a few minor comments as I look though things more closely. Thanks for doing this!

This is super awesome. I like how the tests are set up. Simple and clear with the input/output files.

The only thing I'd change would just be a few function names in the tests to be a bit more verbose to more easily understand at a glance:

    it('should respond properly to a simple non-js file', function(done) {
        const fileName = 'basic-1.html';

        new PrerenderServerInvoker(server.address, testInstance.address).matchFiles(fileName, testFiles.get(fileName)).then(match => {
            assert.equal(match, true);
            done();
        }, done).catch(done);
    });

Since you are starting an express server to serve the input files from a URL, I feel like it reads a little easier like:

    it('should respond properly to a simple non-js file', function(done) {
        const urlName = 'basic-1.html';

        new PrerenderServerInvoker(server.address, testInstance.address).renderURLAndVerifyMatch(urlName, expectedOutputFiles.get(urlName)).then(match => {
            assert.equal(match, true);
            done();
        }, done).catch(done);
    });

It doesn't have to be exactly that, but something like that. Thoughts?

It looks like this doesn't run on Node v10 (I tested with 10.8.0) due to:

  Server - simple
(node:20304) UnhandledPromiseRejectionWarning: TypeError: fs_1.default.promises.opendir is not a function
    at Function.<anonymous> (/Users/todd/projects/prerender/test/utils/OutputLoader.ts:9:41)

Looks like that should be an easy change to get working with Node v10.

It looks like this doesn't run on Node v10 (I tested with 10.8.0) due to:

  Server - simple
(node:20304) UnhandledPromiseRejectionWarning: TypeError: fs_1.default.promises.opendir is not a function
    at Function.<anonymous> (/Users/todd/projects/prerender/test/utils/OutputLoader.ts:9:41)

Looks like that should be an easy change to get working with Node v10.

Right, sorry for that. I was using v14 and forgot to test on v10. Fixed now.

I think the package-lock.json is out of sync after this change and will need to be updated. Shouldn't be an issue but we'll want to make sure those match if we pin these.

I double checked package-lock.json and I couldn't find any discrepancies there. Even deleted it and made npm regenerate it from scratch, and still no changes on my side. I used npm version 6.2.0 which seems to be the bundled npm for node 10.8.0. Can you please give me some more info about what seems to be out of sync on your side?

renderURLAndVerifyMatch

Good call! I did a similar structure now. I still kept the test server URL to be appended automatically to remoteFilePath in the renderTestURLAndVerifyMatch, otherwise we needed to append the server address every time we invoke that call and I didn't want to do that (hence the ...TestURL... fragment in the name).

What do you think?