simplify del/reset commands by using readWriteTruncater interface.

remove the type assertions

update the tests:
the tests now check output as well as file contents separately.

cmd/testhelpers_test.go

@@ -0,0 +1,18 @@
+package cmd
+
+import "bytes"
+
+// seekBuffer wraps bytes.Buffer to satisfy the readWriteTruncater interface,
+// allowing it to be used in place of *os.File in tests.
+type seekBuffer struct {
+	bytes.Buffer
+}
+
+func (s *seekBuffer) Seek(offset int64, whence int) (int64, error) {
+	return 0, nil
+}
+
+func (s *seekBuffer) Truncate(size int64) error {
+	s.Buffer.Reset()
+	return nil
+}