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/middleware/middleware_test.go

@@ -276,10 +276,18 @@ func newTestHandlers(t *testing.T) *handlers.Handlers {
 		t.Fatalf("notify.New: %v", err)
 	}
 
+	st, err := state.New(fxtest.NewLifecycle(t), state.Params{
+		Logger: log,
+		Config: &config.Config{DataDir: t.TempDir()},
+	})
+	if err != nil {
+		t.Fatalf("state.New: %v", err)
+	}
+
 	hnd, err := handlers.New(nil, handlers.Params{
 		Logger:  log,
 		Globals: glob,
-		State:   state.NewForTest(),
+		State:   st,
 		Notify:  notifier,
 	})
 	if err != nil {