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/ircserver/server_test.go

@@ -3,7 +3,6 @@ package ircserver_test
 import (
 	"bufio"
 	"context"
-	"crypto/rand"
 	"database/sql"
 	"errors"
 	"fmt"
@@ -107,14 +106,7 @@ func newTestEnvWithResolver(
 ) *testEnv {
 	t.Helper()
 
-	// A random name, so that no other test environment, not
-	// even an earlier run of the same test, can share it.
-	dsn := fmt.Sprintf(
-		"file:%s?mode=memory&cache=shared",
-		rand.Text(),
-	)
-
-	conn, err := sql.Open("sqlite", dsn)
+	conn, err := sql.Open("sqlite", db.NewTestDatabaseURL(t))
 	if err != nil {
 		t.Fatalf("open db: %v", err)
 	}