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

The test helpers lived in ordinary `testing.go` files inside the config, database, middleware and session packages, so they were built into the binary and the shared `test-support` lint rule could not see them. The four files are gone: the session's helpers move into its own `_test.go` file, and the rest into `configtest`, `databasetest` and `middlewaretest`, which the `depguard` deny list now names, so a non-test file importing them fails lint. The test-support packages build through the production constructors.

Judgement call: the session, the middleware and the webhook database manager now take the plain logger they log through, which the application wiring provides.
Judgement call: two idle-expiry tests move the stored timestamps back instead of advancing a fake clock.

Model: opus-5-5

internal/config/clearenv_test.go

@@ -0,0 +1,37 @@
+package config_test
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/config/configtest"
+)
+
+// TestClearEnv_RemovesAddedVariables pins that a variable set
+// after the clear other than through t.Setenv, as a test's .env file
+// sets one, is gone once the test ends, so it cannot reach the tests
+// that run after it.
+//
+//nolint:paralleltest // ClearEnv uses t.Setenv.
+func TestClearEnv_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.
+	configtest.ClearEnv(t)
+
+	t.Run("loads a .env file after the clear", func(t *testing.T) {
+		configtest.ClearEnv(t)
+
+		path := writeDotEnv(t, dotEnvKey+"=from-dot-env\n")
+		require.NoError(t, config.LoadDotEnvFileForTest(path))
+		require.Equal(t, "from-dot-env", os.Getenv(dotEnvKey))
+	})
+
+	_, present := os.LookupEnv(dotEnvKey)
+	assert.False(
+		t, present,
+		"a variable set after the clear must not outlive the test",
+	)
+}