interfaces/prompting: handle unsupported xattrs

interfaces/prompting: handle unsupported xattrs

This happens in Debian build infrastructure:
```
FAIL: requestrules_test.go:748: requestrulesSuite.TestReadOrAssignUserSessionID

requestrules_test.go:765:
    c.Assert(err, IsNil)
... value syscall.Errno = 0x5f ("operation not supported")
```

Most likely the file system under /run is not a tmpfs or is restricted by a
sandbox of some sort. Allow tests to detect this condition and skip certain
parts.

Signed-off-by: Zygmunt Krynicki <me@zygoon.pl>
Gbp-Pq: Name 0010-interfaces-prompting-handle-unsupported-xattrs.patch