chore: Support running a subset of tests

[rschristian]

It's a tad annoying to work with the test suite as there's effectively no way to focus tests -- at a minimum, this will throw if you haven't built the React transform (which is quite commonly the case for me as I have no reason to touch the React integration).

While not quite Karma's --grep flag, this supports --grep=core,preact,... to optionally filter & focus the test suite.

[changeset-bot[bot]]

⚠️ No Changeset found

Latest commit: 784ac54feb5419b03ec3655573cf66b6413628af

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[netlify[bot]]

✅ Deploy Preview for preact-signals-demo ready!

Name	Link
🔨 Latest commit	784ac54feb5419b03ec3655573cf66b6413628af
🔍 Latest deploy log	https://app.netlify.com/sites/preact-signals-demo/deploys/66f349595cdca0000835ce36
😎 Deploy Preview	https://deploy-preview-600--preact-signals-demo.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[github-actions[bot]]

Size Change: 0 B

Total Size: 83 kB

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `docs/dist/assets/bench.********.js` | 1.59 kB | | `docs/dist/assets/client.********.js` | 46.3 kB | | `docs/dist/assets/index.********.js` | 838 B | | `docs/dist/assets/jsxRuntime.module.********.js` | 284 B | | `docs/dist/assets/preact.module.********.js` | 4.03 kB | | `docs/dist/assets/signals-core.module.********.js` | 1.4 kB | | `docs/dist/assets/signals.module.********.js` | 2.02 kB | | `docs/dist/assets/style.********.js` | 21 B | | `docs/dist/assets/style.********.css` | 1.24 kB | | `docs/dist/basic-********.js` | 244 B | | `docs/dist/demos-********.js` | 3.45 kB | | `docs/dist/nesting-********.js` | 1.13 kB | | `docs/dist/react-********.js` | 242 B | | `packages/core/dist/signals-core.js` | 1.45 kB | | `packages/core/dist/signals-core.mjs` | 1.47 kB | | `packages/preact/dist/signals.js` | 1.27 kB | | `packages/preact/dist/signals.mjs` | 1.22 kB | | `packages/react-transform/dist/signals-*********.js` | 5.03 kB | | `packages/react-transform/dist/signals-transform.mjs` | 4.27 kB | | `packages/react-transform/dist/signals-transform.umd.js` | 5.15 kB | | `packages/react/dist/signals.js` | 188 B | | `packages/react/dist/signals.mjs` | 150 B |

_{compressed-size-action}