Preserve user comments when editing `fmf` files

[psss]

Several tmt commands are using the fmf.Tree context manager to store modified node data to disk. It would be nice to use the rt round trip mode to preserve user comments in a similar way as in teemtee/tmt/pull/4712.

[LecrisUT]

Small note on this. fmf must provide the read-write interface in that case because if we accidentally mix the ruamel.yaml load/dump modes it blows up in our face (e.g. fmf does the yaml load with rt, but tmt dumps the fmf tree with safe)