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/database/retention_test.go

@@ -51,7 +51,7 @@ func setupRetentionTest(t *testing.T) *retentionTestEnv {
 
 	mgr, err := database.NewWebhookDBManager(
 		lc,
-		database.WebhookDBManagerParams{Config: cfg, Logger: l},
+		database.WebhookDBManagerParams{Config: cfg, Logger: l.Get()},
 	)
 	require.NoError(t, err)