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.

The new package internal/dbtest has NewURL. It returns the URL of a
new in-memory database with a random name and holds one connection to
it open until the test ends. The db, handlers, ircserver, server and
service tests take their database URL from it. The test-support lint
rule keeps the package out of the server binary.

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

README.md

@@ -3342,6 +3342,7 @@ neoirc/
 │   │   └── schema/
 │   │       ├── 000.sql     # schema_migrations bootstrap
 │   │       └── 001_initial.sql
+│   ├── dbtest/             # Gives each test its own database; only tests import it
 │   ├── globals/            # Application-wide metadata
 │   │   └── globals.go
 │   ├── handlers/           # HTTP request handlers