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)))))

• harness holds a session's islands under a root: an island's call, such as (app uid), or a fn of no arguments returning one, called for each render. Its options are the session's :uid ("user" by default) and :tab, an :error-view as the app's, and the :request that created the session, which use-session returns and the handler's action/request returns too. Without a request, action/request throws, as it does outside an action.
• 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.
• mounted lists the ids of the mounted islands, the paths of slots from the root.
• (action h path k) is the action the island at path renders under k, 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.
• failures lists the renders that threw, which the page would show only as an error view: each the failure the view got, with the island's id under :path and the exception under :error, whichever view rendered it. An island failing again the same way repeats its :ref.
• 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 Scoped's 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.

Harnesses share an island definition's stack traces in the log, as sessions do (Serving pages). So an island failing the same way in several tests within a minute may log a line naming an earlier reference, rather than the exception. Assert on test/failures, which has each render's exception.