Keep each test database when the pool replaces its connection (closes #209)

SQLite deletes a shared in-memory database when its last connection
closes. A query cancelled while it runs makes the pool close its
connection, so a test could lose all its tables and rows at random.

NewTestDatabaseURL in internal/db returns the URL of a new in-memory
database with a random name. It holds one connection to it open until
the test ends. The db, handlers, ircserver, server and service tests
now take their database URL from it.

A new test cancels a query on such a database, checks with a TEMP
table that the connection was replaced, and checks that a row is
still there.

Model: opus-5-5

internal/server/server_test.go

@@ -5,7 +5,6 @@
 package server_test
 
 import (
-	"crypto/rand"
 	"fmt"
 	"net"
 	"net/http"
@@ -241,10 +240,7 @@ func newApp(
 			stats.New,
 		),
 		fx.Decorate(func(cfg *config.Config) *config.Config {
-			// A random name, so that no other test can share it.
-			cfg.DBURL = fmt.Sprintf(
-				"file:%s?mode=memory&cache=shared", rand.Text(),
-			)
+			cfg.DBURL = db.NewTestDatabaseURL(t)
 			cfg.Port = port
 
 			return cfg