Keep test helpers out of the shipped binary (closes #506)

The four testing.go files in config, database, middleware and session
are gone. ClearEnvForTest moves to internal/config/configtest; the
webhook database manager helpers move to internal/database/databasetest
and the middleware's NewForTest to internal/middleware/middlewaretest,
both now built through the production constructors. Tests that wrapped
an open main database use database.Open. The session helpers move into
the session package's export_test.go; the middleware tests build their
session through session.New and age its timestamps instead of using a
fake clock.

The session, middleware and webhook database manager now take the
*slog.Logger they log through, so tests in other packages can give
them their own.

Model: opus-5-5

internal/middleware/metrics.go

@@ -151,7 +151,7 @@ var _ httpmetrics.Recorder = boundedLabelRecorder{}
 
 // Metrics returns middleware that records Prometheus HTTP metrics
 // with the Middleware's one recorder, which New builds on the registry
-// the /metrics route serves and NewForTest on a registry of its own.
+// it is given: in the application, the one the /metrics route serves.
 // Every call reuses that recorder, so any number of routers can
 // install it.
 func (s *Middleware) Metrics() func(http.Handler) http.Handler {