test: add tests for hot middleware

Covers schema validation of the `hot` option (success and failure cases
with snapshots), unit tests for `pathMatch`, `formatErrors`,
`buildModuleMap` and `createEventStream`, and integration tests that
verify SSE headers, the default and custom hot paths, MultiCompiler
support, `close()` teardown, and the `log` option (custom function and
`log: false`).

Author: bjohansebas

test/__snapshots__/validation-options.test.js.snap.webpack5

@@ -75,6 +75,63 @@ exports[`validation should throw an error on the "headers" option with "true" va
     * options.headers should be an instance of function."
 `;
 
+exports[`validation should throw an error on the "hot" option with "{"heartbeat":-1}" value 1`] = `
+"Invalid options object. Dev Middleware has been initialized using an options object that does not match the API schema.
+ - options.hot.heartbeat should be >= 0.
+   -> Heartbeat interval (in milliseconds) used to keep the SSE connection alive."
+`;
+
+exports[`validation should throw an error on the "hot" option with "{"log":true}" value 1`] = `
+"Invalid options object. Dev Middleware has been initialized using an options object that does not match the API schema.
+ - options.hot should be one of these:
+   boolean | object { path?, heartbeat?, log?, statsOptions? }
+   -> Enable hot module replacement via a Server-Sent Events endpoint.
+   -> Read more at https://github.com/webpack/webpack-dev-middleware#hot
+   Details:
+    * options.hot.log should be one of these:
+      false | function
+      -> Logger used to print build status. Pass \`false\` to disable.
+      Details:
+       * options.hot.log should be false.
+       * options.hot.log should be an instance of function."
+`;
+
+exports[`validation should throw an error on the "hot" option with "{"path":""}" value 1`] = `
+"Invalid options object. Dev Middleware has been initialized using an options object that does not match the API schema.
+ - options.hot.path should be a non-empty string.
+   -> The path the SSE endpoint is served at."
+`;
+
+exports[`validation should throw an error on the "hot" option with "{"unknown":true}" value 1`] = `
+"Invalid options object. Dev Middleware has been initialized using an options object that does not match the API schema.
+ - options.hot has an unknown property 'unknown'. These properties are valid:
+   object { path?, heartbeat?, log?, statsOptions? }"
+`;
+
+exports[`validation should throw an error on the "hot" option with "0" value 1`] = `
+"Invalid options object. Dev Middleware has been initialized using an options object that does not match the API schema.
+ - options.hot should be one of these:
+   boolean | object { path?, heartbeat?, log?, statsOptions? }
+   -> Enable hot module replacement via a Server-Sent Events endpoint.
+   -> Read more at https://github.com/webpack/webpack-dev-middleware#hot
+   Details:
+    * options.hot should be a boolean.
+    * options.hot should be an object:
+      object { path?, heartbeat?, log?, statsOptions? }"
+`;
+
+exports[`validation should throw an error on the "hot" option with "foo" value 1`] = `
+"Invalid options object. Dev Middleware has been initialized using an options object that does not match the API schema.
+ - options.hot should be one of these:
+   boolean | object { path?, heartbeat?, log?, statsOptions? }
+   -> Enable hot module replacement via a Server-Sent Events endpoint.
+   -> Read more at https://github.com/webpack/webpack-dev-middleware#hot
+   Details:
+    * options.hot should be a boolean.
+    * options.hot should be an object:
+      object { path?, heartbeat?, log?, statsOptions? }"
+`;
+
 exports[`validation should throw an error on the "index" option with "{}" value 1`] = `
 "Invalid options object. Dev Middleware has been initialized using an options object that does not match the API schema.
  - options.index should be one of these: