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/config/testing_test.go

@@ -7,6 +7,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/config/configtest"
 )
 
 // TestClearEnvForTest_RemovesAddedVariables pins that a variable set
@@ -18,10 +19,10 @@ import (
 func TestClearEnvForTest_RemovesAddedVariables(t *testing.T) {
 	// The outer clear keeps a value of the key exported in the shell
 	// from making it a variable the inner clear has to put back.
-	config.ClearEnvForTest(t)
+	configtest.ClearEnvForTest(t)
 
 	t.Run("loads a .env file after the clear", func(t *testing.T) {
-		config.ClearEnvForTest(t)
+		configtest.ClearEnvForTest(t)
 
 		path := writeDotEnv(t, dotEnvKey+"=from-dot-env\n")
 		require.NoError(t, config.LoadDotEnvFileForTest(path))