tests: move the test-only constructors into export_test.go (closes #111)

state.NewForTest, state.NewForTestWithDataDir and watcher.NewForTest
were compiled into and exported by the production packages
internal/state and internal/watcher. NewForTestWithDataDir and
watcher.NewForTest now live in their package's export_test.go. Tests in
other packages cannot see those files, so the watcher and middleware
tests build their State with state.New and a temporary data directory;
the watcher tests no longer try to save to /state.json.
state.NewForTest, whose State saved to /, is deleted: the state tests
that used it pass t.TempDir() to NewForTestWithDataDir, and the test of
the helper itself is gone.

Model: opus-5-5

internal/watcher/watcher_test.go

@@ -9,8 +9,12 @@ import (
 	"testing"
 	"time"
 
+	"go.uber.org/fx/fxtest"
+
 	"sneak.berlin/go/dnswatcher/internal/config"
+	"sneak.berlin/go/dnswatcher/internal/globals"
 	"sneak.berlin/go/dnswatcher/internal/livednstest"
+	"sneak.berlin/go/dnswatcher/internal/logger"
 	"sneak.berlin/go/dnswatcher/internal/portcheck"
 	"sneak.berlin/go/dnswatcher/internal/resolver"
 	"sneak.berlin/go/dnswatcher/internal/state"
@@ -148,7 +152,24 @@ func newTestWatcher(
 		config:   cfg,
 	}
 
-	deps.state = state.NewForTest()
+	g, err := globals.New(nil)
+	if err != nil {
+		t.Fatalf("globals.New: %v", err)
+	}
+
+	log, err := logger.New(nil, logger.Params{Globals: g})
+	if err != nil {
+		t.Fatalf("logger.New: %v", err)
+	}
+
+	// The watcher saves state after every check, into cfg.DataDir.
+	deps.state, err = state.New(fxtest.NewLifecycle(t), state.Params{
+		Logger: log,
+		Config: cfg,
+	})
+	if err != nil {
+		t.Fatalf("state.New: %v", err)
+	}
 
 	w := watcher.NewForTest(
 		deps.config,