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

state.NewForTest, state.NewForTestWithDataDir and watcher.NewForTest
were in ordinary source files, so they were built into the binary.
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/state/export_test.go

@@ -0,0 +1,23 @@
+package state
+
+import (
+	"log/slog"
+
+	"sneak.berlin/go/dnswatcher/internal/config"
+)
+
+// NewForTestWithDataDir creates an empty State that saves to dataDir,
+// without the fx lifecycle.
+func NewForTestWithDataDir(dataDir string) *State {
+	return &State{
+		log: slog.Default(),
+		snapshot: &Snapshot{
+			Version:      stateVersion,
+			Domains:      make(map[string]*DomainState),
+			Hostnames:    make(map[string]*HostnameState),
+			Ports:        make(map[string]*PortState),
+			Certificates: make(map[string]*CertificateState),
+		},
+		config: &config.Config{DataDir: dataDir},
+	}
+}