fix failing test about error

This line
// {"error: %s", []interface{}{errorToBePrinted}, "error: can I be printed?"},

gave panic: reflect: reflect.Value.SetString using value obtained using unexported field

Seems to have to do with the for loop using []interface{} and
what was being done in the body of the for.

in gnovm/stdlibs/error, exporting s in errorString fixed the error:

type errorString struct { S /* <- this */ string }

But moved the test in its own function, this passes:

```
func TestPrintErrors(t *testing.T) {
got := Sprintf("error: %s", errors.New("can I be printed?"))
expectedOutput := "error: can I be printed?"
if got != expectedOutput {
    t.Errorf("got %q, want %q.", got, expectedOutput)
}
}
```

examples/gno.land/p/demo/ufmt/ufmt_test.gno

@@ -13,7 +13,6 @@ func (stringer) String() string {
 }
 
 func TestSprintf(t *testing.T) {
-	errorToBePrinted := errors.New("can I be printed?")
 	tru := true
 	cases := []struct {
 		format         string
@@ -42,7 +41,6 @@ func TestSprintf(t *testing.T) {
 		{"â", nil, "â"},
 		{"Hello, World! 😊", nil, "Hello, World! 😊"},
 		{"unicode formatting: %s", []interface{}{"😊"}, "unicode formatting: 😊"},
-		{"error: %s", []interface{}{errorToBePrinted}, "error: can I be printed?"},
 		// mismatch printing
 		{"%s", []interface{}{nil}, "%!s(<nil>)"},
 		{"%s", []interface{}{421}, "%!s(int=421)"},
@@ -129,6 +127,14 @@ func TestErrorf(t *testing.T) {
 	}
 }
 
+func TestPrintErrors(t *testing.T) {
+	got := Sprintf("error: %s", errors.New("can I be printed?"))
+	expectedOutput := "error: can I be printed?"
+	if got != expectedOutput {
+		t.Errorf("got %q, want %q.", got, expectedOutput)
+	}
+}
+
 // NOTE: Currently, there is no way to get the output of Println without using os.Stdout,
 // so we can only test that it doesn't panic and print arguments well.
 func TestPrintln(t *testing.T) {