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/env_test.go

@@ -7,6 +7,7 @@ import (
 	"github.com/stretchr/testify/require"
 	"go.uber.org/fx"
 	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/config/configtest"
 	"sneak.berlin/go/webhooker/internal/globals"
 	"sneak.berlin/go/webhooker/internal/logger"
 )
@@ -120,7 +121,7 @@ func TestEnvBool(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			// Cannot use t.Parallel() here because t.Setenv
 			// is incompatible with parallel subtests.
-			config.ClearEnvForTest(t)
+			configtest.ClearEnvForTest(t)
 
 			if tt.set {
 				t.Setenv(testEnvKey, tt.value)
@@ -169,7 +170,7 @@ func runEnvIntCases(
 		t.Run(tt.name, func(t *testing.T) {
 			// Cannot use t.Parallel() here because t.Setenv
 			// is incompatible with parallel subtests.
-			config.ClearEnvForTest(t)
+			configtest.ClearEnvForTest(t)
 
 			if tt.set {
 				t.Setenv(testEnvKey, tt.value)
@@ -310,7 +311,7 @@ func TestEnvBindAddress(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			// Cannot use t.Parallel() here because t.Setenv
 			// is incompatible with parallel subtests.
-			config.ClearEnvForTest(t)
+			configtest.ClearEnvForTest(t)
 
 			if tt.set {
 				t.Setenv(testEnvKey, tt.value)
@@ -476,7 +477,7 @@ func TestNewRejectsBadEnvValues(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			// Cannot use t.Parallel() here because t.Setenv
 			// is incompatible with parallel subtests.
-			config.ClearEnvForTest(t)
+			configtest.ClearEnvForTest(t)
 			t.Setenv("WEBHOOKER_ENVIRONMENT", "dev")
 			t.Setenv(tt.key, tt.value)
 
@@ -638,7 +639,7 @@ func sentryEnvValueCases() []badEnvValueCase {
 // break the legitimate unset case: absent variables still get their
 // documented defaults.
 func TestNewUsesDefaultsWhenUnset(t *testing.T) {
-	config.ClearEnvForTest(t)
+	configtest.ClearEnvForTest(t)
 	t.Setenv("WEBHOOKER_ENVIRONMENT", "dev")
 
 	cfg, err := buildConfig(t)