Testing islands

co.multiply.tropical.test renders an app's islands as a session does, on the test's own thread, with no server, stream or browser:

(deftest clicking-counts
  (with-open [h (test/harness #(app uid) {:uid uid})]
    (is (str/includes? (test/render! h) "Clicked 0 times"))
    ((test/action h "app/counter" :click) {})
    (is (str/includes? (test/render! h) "Clicked 1 times"))
    (is (empty? (test/failures h)))))

• render! renders what changed since the last render, and returns the HTML of the whole tree. While a read is pending, as an observe whose source hasn't emitted yet, it waits for it, up to :wait-ms (1 s by default), so a test sees what the read brought. {:wait-ms 0} renders once, for what the island shows while it waits.
• action is the action an island renders under a key, found by the island's id, the path of slots from the root that mounted lists, as a fn of the signals, and of a value for an action taking one. It is nil while the island doesn't render it, which is how a test sees a gate. The signals pass through JSON, as the browser's do, so the handler gets string keys. With :request in the harness's options, use-session has it as the request that created the session, and the handler's action/request returns it.
• failures lists the renders that threw, with their exceptions, which the page would show only as the error view.
• Closing the harness unmounts its islands: their resources start to linger and their action tokens are revoked.
• Renders run in the caller's scope. Bind with scoping what the route's middleware would bind for a session.
• Sources are the app's. An island reading through the app's own function over observe reads what that function opens, so a test that shouldn't reach a database redefines the function, with with-redefs, to one over a source of the test's own.