pkg/md/fmt_test.go: Remove test for the idempotency of FmtCodec.
pkg/md/fmt_test.go: Remove test for the idempotency of FmtCodec.

It is redundant for the existing check that the HTML output of formatted
markdown remains the same.

elves/elvish,elves/elvish,elves/elvish,elves/elvish,elves/elvish