Change `--junit-xml=path` to accept directories or change help text.

[decentral1se]

I was working on #595 and ran into this.

When running:

$ pytest --junit-xml=.

I receive a IsADirectoryError: [Errno 21] error.

What I need to pass to make this work is:

$ pytest --junit-xml=somefile

Could --junit-xml=. default to create some report file?

Otherwise, should the help text be altered to highlight that it must be a file name?

[nicoddemus]

I think we should warn the user that the option must be a filename and not an existing directory, similar that what was done in #2080.

The check would have to reside somewhere in junitxml.py thought... I wonder if argparse doesn't support some validation mechanism for this instead?

[decentral1se]

I wonder if argparse doesn't support some validation mechanism for this instead?

Well, it seems that argparse gives us a type kwarg.

Check the diff in #2097, if it isn't acceptable, I can add the specific warning.