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/handlers/api_test.go

@@ -3,7 +3,6 @@ package handlers_test
 import (
 	"bytes"
 	"context"
-	"crypto/rand"
 	"database/sql"
 	"encoding/json"
 	"errors"
@@ -164,13 +163,7 @@ func newTestServerWith(
 	viperMu.Lock()
 	defer viperMu.Unlock()
 
-	// 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)
 	copyTemplateDB(t, dbURL)
 
 	var (