Add example for how to test any error is thrown in the error testing documentation article (#853)

I was stumped earlier when trying to find the details for how to test
that any error is thrown. This detail _is_ in the migrating from XCTest
content, but I landed on this page, and it seemed an odd miss,
especially given the history of ignoring the kind of error in many
cases. (I found my solution in [a post by Jonathan the
forums](https://forums.swift.org/t/swift-testing-whats-the-recommend-approach-to-test-throwing-function/70806/2).)

While it is duplicated content, this would have helped me - so I'm
proposing it for a documentation update. Happy to edit as y'all see fit.

---------

Co-authored-by: Stuart Montgomery <[email protected]>

Author: heckj

Sources/Testing/Testing.docc/testing-for-errors-in-swift-code.md

@@ -44,6 +44,22 @@ test that the code throws an error of a given type, or matches an arbitrary
 Boolean test. Similar overloads of ``require(_:_:sourceLocation:)-5l63q`` stop
 running your test if the code doesn't throw the expected error.
 
+### Validate that your code throws any error
+
+To check that the code under test throws an error of any type, pass
+`(any Error).self` as the first argument to either
+``expect(throws:_:sourceLocation:performing:)-1xr34`` or
+``require(_:_:sourceLocation:)-5l63q``:
+
+```swift
+@Test func cannotAddToppingToPizzaBeforeStartOfList() {
+  var order = PizzaToppings(bases: [.calzone, .deepCrust])
+  #expect(throws: (any Error).self) {
+    try order.add(topping: .mozarella, toPizzasIn: -1..<0)
+  }
+}
+```
+
 ### Validate that your code doesn't throw an error
 
 A test function that throws an error fails, which is usually sufficient for