doc: add missing function to test common doc

Add documentation for `common.expectsError()` to `test/README.md`.

PR-URL: #11382
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/README.md

@@ -180,6 +180,22 @@ Platform normalizes the `dd` command
 
 Check if there is more than 1gb of total memory.
 
+### expectsError(code[, type[, message]])
+* `code` [<String>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type)
+  expected error must have this value for its `code` property
+* `type` [<Function>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function)
+  expected error must be an instance of `type`
+* `message` [<String>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type)
+  or [<RegExp>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp)
+  if a string is provided for `message`, expected error must have it for its
+  `message` property; if a regular expression is provided for `message`, the
+  regular expression must match the `message` property of the expected error
+
+* return function suitable for use as a validation function passed as the second
+  argument to `assert.throws()`
+
+The expected error should be [subclassed by the `internal/errors` module](https://github.com/nodejs/node/blob/master/doc/guides/using-internal-errors.md#api).
+
 ### expectWarning(name, expected)
 * `name` [<String>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type)
 * `expected` [<String>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type) | [<Array>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array)