Test adding a non-empty file to testdata when test expects empty files

-- p_test.go --
package p

import (
	"os"
	"path/filepath"
	"testing"
)

func TestEmptyTestdata(t *testing.T) {
	files, _ := filepath.Glob("testdata/*")
	for _, path := range files {
		data, _ := os.ReadFile(path)
		if len(data) > 0 {
			t.Fatalf("testdata file is not empty: %s", path)
		}
	}
}

-- testdata/empty --
-- .commit --
Initial commit with empty testdata file

-- testdata/nonempty --
hi
-- .commit --
Add non-empty file to testdata

-- .run_test --
Test regressions detected between initial commit (cdc9ec6dfb8669c11d8aa0df49c72112627784dc) and HEAD:

1: sketch.dev.TestEmptyTestdata: Was passing, now failing


Please fix before proceeding.