[Perl/perl5] b19fe3: op/exec.t: ENOTDIR is a legit errno

[Karl Williamson]

  Branch: refs/heads/smoke-me/khw-enotdir
  Home:   https://github.com/Perl/perl5
  Commit: b19fe3468fb52a5f452f34f310efb16fe87d9693
      
https://github.com/Perl/perl5/commit/b19fe3468fb52a5f452f34f310efb16fe87d9693
  Author: Karl Williamson <k...@cpan.org>
  Date:   2020-04-09 (Thu, 09 Apr 2020)

  Changed paths:
    M t/op/exec.t

  Log Message:
  -----------
  op/exec.t: ENOTDIR is a legit errno

If PATH contains a component that isn't a directory, say non-existent or
is instead a file, this errno can be returned in addition to the ones it
already checked for.

I examined the possible errnos listed in a Linux man page for this, and
this looked to be the only one that is likely to come up that weren't
already covered.

This fixes GH #17515