[Bug libstdc++/81808] 27_io /basic_fstream/53984.cc failure on AIX

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81808

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
On AIX reading the directory just reads the actual contents of the directory,
i.e. the files contained in the directory. Maybe we should restrict the test to
only run on targets where that read used to fail with an exception.

Alternatively, we could read a char instead of an int, and if the read succeeds
don't test for in.bad(). I'd rather just restrict the targets where it runs
though.