Make the load-sensitive tests wait for what they check (closes #507)

Three tests failed at random on a busy host. The browser test now waits for each page a click opens to load, and for Alpine.js to start on it, before reading it. The delivery tests' drain takes what is already queued instead of racing a 25 ms timer, since both dispatch paths queue before they return. The test phase keeps the tests' temporary directories on a tmpfs, because SQLite waiting for the disk made `internal/handlers` slow under load; the timeout and the parallel cap are unchanged.

Deviation: the delivery tests do not wait with a deadline; dispatch has finished when they read.
Unverified: `internal/handlers` at a host load of 260 to 290.

Model: opus-5-5

internal/delivery/inflight_test.go

@@ -48,8 +48,9 @@ func fSweepSetup(
 //
 // Every caller drives the dispatch paths synchronously and has already
 // waited for them to return, so anything they queued is in the channel
-// by now. The short grace covers nothing but scheduler jitter, and is
-// kept small because one of these tests runs the drain forty times.
+// by now, and nothing is waited for. A timer here would race the queued
+// tasks: on a busy host it can be due by the time select looks, and
+// select picks at random among the cases that are ready.
 func fDrain(e *delivery.Engine) []delivery.Task {
 	var out []delivery.Task
 
@@ -59,7 +60,7 @@ func fDrain(e *delivery.Engine) []delivery.Task {
 			out = append(out, task)
 		case task := <-e.ExportRetryCh():
 			out = append(out, task)
-		case <-time.After(25 * time.Millisecond):
+		default:
 			return out
 		}
 	}