Change .html to .https.html, add tests for [SecureContext] (#809)

* Use .https.html files

WPT uses the filname `*.https.html` to determine to run the test over an
HTTPS connection. This is now required because WebGPU requires
[SecureContext]: https://github.com/gpuweb/gpuweb/pull/1363

* Add tests for [SecureContext]
diff --git a/src/webgpu/examples.spec.ts b/src/webgpu/examples.spec.ts
index 908fbe8..63d83fb 100644
--- a/src/webgpu/examples.spec.ts
+++ b/src/webgpu/examples.spec.ts
@@ -11,7 +11,7 @@
 // To run these tests in the standalone runner, run `npm start` then open:
 // - http://localhost:XXXX/standalone/?runnow=1&q=webgpu:examples:*
 // To run in WPT, copy/symlink the out-wpt/ directory as the webgpu/ directory in WPT, then open:
-// - (wpt server url)/webgpu/cts.html?q=webgpu:examples:
+// - (wpt server url)/webgpu/cts.https.html?q=webgpu:examples:
 //
 // Tests here can be run individually or in groups:
 // - ?q=webgpu:examples:basic,async: