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/handlers/target_create_query_test.go

@@ -15,7 +15,7 @@ import (
 	"github.com/stretchr/testify/require"
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/database"
-	"sneak.berlin/go/webhooker/internal/middleware"
+	"sneak.berlin/go/webhooker/internal/middleware/middlewaretest"
 )
 
 // targetSecretSegments are the path segments of an incoming-webhook
@@ -65,7 +65,8 @@ func postTargetCreate(
 	t.Helper()
 
 	logBuf := new(bytes.Buffer)
-	mw := middleware.NewForTest(
+	mw := middlewaretest.NewForTest(
+		t,
 		slog.New(slog.NewJSONHandler(
 			logBuf, &slog.HandlerOptions{Level: slog.LevelInfo},
 		)),