Add tests for dircache updates

[t-w]

"dircache updates possible bug(s)? "

It was mentioned in README.md, but no more info.

(to check/investigate)

[t-w]

@lclevy, any idea what was this about? there are no more details about it...

[lclevy]

It is 30 years ago :-)
As I remember it was not tested enough

[t-w]

OK. Thanks.

I will add the info to the notes about the write mode in README.