refactor: move testfixtures to internal package and update imports

- Relocated testfixtures from the root to the internal package for better encapsulation.
- Updated import paths in codec test files across env, ini, json, yaml directories to reflect the new location.
- Removed the now obsolete testfixtures.go file.

internal/testfixtures/testfixtures.go

@@ -0,0 +1,46 @@
+// Package testfixtures resolves paths to files under the module's fixtures/ directory.
+package testfixtures
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+func moduleRoot(t *testing.T) string {
+	t.Helper()
+	_, file, _, ok := runtime.Caller(0)
+	if !ok {
+		t.Fatal("runtime.Caller")
+	}
+	dir := filepath.Dir(file)
+	for {
+		data, err := os.ReadFile(filepath.Join(dir, "go.mod"))
+		if err == nil && strings.Contains(string(data), "module github.com/eslider/go-config") {
+			return dir
+		}
+		parent := filepath.Dir(dir)
+		if parent == dir {
+			t.Fatal("module root not found")
+		}
+		dir = parent
+	}
+}
+
+// Path joins repo fixtures/ with parts.
+func Path(t *testing.T, parts ...string) string {
+	t.Helper()
+	return filepath.Join(append([]string{moduleRoot(t), "fixtures"}, parts...)...)
+}
+
+// Load reads a fixture file relative to fixtures/.
+func Load(t *testing.T, parts ...string) []byte {
+	t.Helper()
+	b, err := os.ReadFile(Path(t, parts...))
+	if err != nil {
+		t.Fatal(err)
+	}
+	return b
+}