Add test to ensure that file icons are one rune

This should prevent errors like that from happening again.

pkg/gui/presentation/icons/file_icons_test.go

@@ -0,0 +1,21 @@
+package icons
+
+import (
+	"testing"
+)
+
+func TestFileIcons(t *testing.T) {
+	t.Run("TestFileIcons", func(t *testing.T) {
+		for name, icon := range nameIconMap {
+			if len([]rune(icon.Icon)) != 1 {
+				t.Errorf("nameIconMap[\"%s\"] is not a single rune", name)
+			}
+		}
+
+		for ext, icon := range extIconMap {
+			if len([]rune(icon.Icon)) != 1 {
+				t.Errorf("extIconMap[\"%s\"] is not a single rune", ext)
+			}
+		}
+	})
+}