Add template snapshots

[drwpow]

We recently had some issues with our Snowpack templates, so we want to have a more robust test suite.

Before

• We made sure the npx create-snowpack-app command built something with all the templates, but we didn’t check what it built.

After

• We run through each template, and test its build against a snapshot
• The test suite throws if a template doesn’t match its snapshot (at which point it’s up to the author + reviewer to decide whether this is an improvement or a regression.)
• We still test npx create-snowpack-app, but now only for 1 template (which seems to be fine)

Suggestion

If we added dependabot to this repo, which bumped PRs on Snowpack releases, then we’d have really darn good coverage as the snowpack dependency is the only thing we’re not able to test as part of this.

[drwpow]

Looks like Windows is being Windows again.

Preact/React:

- "export default "\\_dist_\\logo.png";"
+ "export default "/_dist_/logo.png";"

Svelte:

- /* src\App.svelte generated by Svelte v3.23.2 */
+ /* src/App.svelte generated by Svelte v3.23.2 */

🤔 I don’t want to hide this, as I do want to fix this. But I’m not sure if there’s an easy fix for it in this PR that adds these tests.

[FredKSchott]

That Preact issue looks bad though, wouldn't that be broken on Windows?

Yea, I'm not sure what the right move is here. We've used strip-comments in the past, possibly even for this exact reason. I'd be alright with stripping comments from what we compare against, since a broken / separator is fine in a comment but actually broken in the code itself.

[drwpow]

That Preact issue looks bad though, wouldn't that be broken on Windows?

I think so. I actually think it has to do with the import rewriting. Not unique to React or Preact; it’s in Snowpack. It’s just appeared here because I don’t think we have a test for it in Snowpack. Fixing there would be better.

[FredKSchott]

Hmm, that's surprising that that's not covered by our current test suite.

I'd vote to fix now in Snowpack, or skip this test until we can fix. No need to work around a bug here :)

[drwpow]

Merging! This doesn’t affect the templates themselves; this just strengthens this repo with a better test suite, as well as a better test environment for Snowpack itself (now you can run npm run build && cp -r pkg/* ~/path/to/create-snowpack-app locally, run yarn test in this repo, and you’ll be able to test Snowpack build pretty easily)