fix(createTestServer): fix the .js MIME type (#27)

The proper MIME type for JavaScript files is "text/javascript", not
"application/javascript". Even though "application/javascript" is on the
list of JavaScript MIME types:
https://mimesniff.spec.whatwg.org/#javascript-mime-type per:
https://html.spec.whatwg.org/multipage/scripting.html#scriptingLanguages

> Servers should use text/javascript for JavaScript resources, in
accordance
> with Updates to ECMAScript Media Types. Servers should not use other
> JavaScript MIME types for JavaScript resources

"Updates to ECMAScript Media Types" is the following RFC:
https://www.rfc-editor.org/rfc/rfc9239

In section "2. Compatibility" it says:
> The most widely supported media type in use is text/javascript; all
others
> are considered historical and obsolete aliases of text/javascript.

Author: mgol

createTestServer.js

@@ -145,7 +145,7 @@ export async function createTestServer( {
 	const validMimeTypes = {
 
 		// No .mjs or .cjs files are used in tests
-		".js": "application/javascript",
+		".js": "text/javascript",
 		".json": "application/json",
 		".css": "text/css",
 		".html": "text/html",