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/service/service_test.go

@@ -6,7 +6,6 @@ package service_test
 
 import (
 	"context"
-	"crypto/rand"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -46,13 +45,7 @@ type testEnv struct {
 func newTestEnv(t *testing.T) *testEnv {
 	t.Helper()
 
-	// A random name, so that no other test can share it, not
-	// even one that has finished but still holds its database
-	// open.
-	dbURL := fmt.Sprintf(
-		"file:%s?mode=memory&cache=shared",
-		rand.Text(),
-	)
+	dbURL := db.NewTestDatabaseURL(t)
 
 	var (
 		database *db.Database