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/db/export_test.go

@@ -5,21 +5,14 @@ import (
 	"database/sql"
 	"fmt"
 	"log/slog"
-	"sync/atomic"
+	"testing"
 )
 
-//nolint:gochecknoglobals // test counter
-var testDBCounter atomic.Int64
-
 // NewTestDatabase creates an in-memory database for testing.
-func NewTestDatabase() (*Database, error) {
-	counter := testDBCounter.Add(1)
+func NewTestDatabase(tb testing.TB) (*Database, error) {
+	tb.Helper()
 
-	dsn := fmt.Sprintf(
-		"file:testdb%d?mode=memory"+
-			"&cache=shared&_pragma=foreign_keys(1)",
-		counter,
-	)
+	dsn := NewTestDatabaseURL(tb) + "&_pragma=foreign_keys(1)"
 
 	conn, err := sql.Open("sqlite", dsn)
 	if err != nil {