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/dotenv_test.go

@@ -8,6 +8,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/config/configtest"
 )
 
 // dotEnvKey is a throwaway variable name the .env tests write and
@@ -41,7 +42,7 @@ func writeDotEnv(t *testing.T, contents string) string {
 //
 //nolint:paralleltest // ClearEnvForTest uses t.Setenv.
 func TestLoadDotEnv_MissingFileIsFine(t *testing.T) {
-	config.ClearEnvForTest(t)
+	configtest.ClearEnvForTest(t)
 
 	absent := filepath.Join(t.TempDir(), config.DotEnvPath)
 	require.NoError(t, config.LoadDotEnvFileForTest(absent))
@@ -56,7 +57,7 @@ func TestLoadDotEnv_MissingFileIsFine(t *testing.T) {
 //
 //nolint:paralleltest // ClearEnvForTest uses t.Setenv.
 func TestLoadDotEnv_AppliesValues(t *testing.T) {
-	config.ClearEnvForTest(t)
+	configtest.ClearEnvForTest(t)
 
 	path := writeDotEnv(t, "# a comment\n"+dotEnvKey+"=from-dot-env\n")
 
@@ -84,7 +85,7 @@ func TestLoadDotEnv_RealEnvironmentWins(t *testing.T) {
 //
 //nolint:paralleltest // ClearEnvForTest uses t.Setenv.
 func TestLoadDotEnv_MalformedFileAborts(t *testing.T) {
-	config.ClearEnvForTest(t)
+	configtest.ClearEnvForTest(t)
 
 	path := writeDotEnv(
 		t, malformedDotEnv+dotEnvKey+"=from-dot-env\n",
@@ -132,7 +133,7 @@ func TestLoadDotEnv_UnreadableFileAborts(t *testing.T) {
 //
 //nolint:paralleltest // t.Chdir moves the whole process.
 func TestLoadDotEnv_ReadsTheWorkingDirectory(t *testing.T) {
-	config.ClearEnvForTest(t)
+	configtest.ClearEnvForTest(t)
 
 	dir := t.TempDir()
 	require.NoError(t, os.WriteFile(