Fix tar.gz extraction for regressions testsuite file

[strarsis]

The SVG test suite tar.gz file appears to be also gunzip-compressed, hence tar-stream alone used in the current regression test setup fails to extract the file, hence the whole regressions test fails:
Download W3C SVG 1.1 Test Suite and extract svg files Error: Invalid tar header. Maybe the tar is corrupted or it needs to be gunzipped?

This PR adds gunzip-maybe in between that can handle gunzip-compressed archives (as recommended by the tar-stream docs).

[SethFalco]

I think instead of adding the dev dependency, it'd be better for us to just take the one function call that actually matters.

        swap(null, pumpify(zlib.createGunzip(), gunzip(maxRecursion - 1)))

— https://github.com/mafintosh/gunzip-maybe/blob/master/index.js#L21

The rest is just various checks or determining whether calling zlib#createGunzip is required or not. As we're hard-coding the URL, I think we can handle things ourselves.

Based on this, both review comments are redundant, but I thought it was worth sharing the FYI anyway! As I'd already tested this approach locally, I've created a separate PR for it here:

• test: fix regression testing action #1788

[SethFalco]

This belongs in devDependencies!
When adding it, you'd do:

yarn add gunzip-maybe --dev`

[SethFalco]

I'm guessing you used npm to install the new dependency. This project uses Yarn, so normally you should manage dependencies with Yarn instead!

You'll be able to tell because there is a yarn.lock file in the root of the repository.