User perms are not checked on file access

[toddr]

Currently we don't check perms with file access. Given we're providing modes, we might as well check current UID/GID and do the right fail if necessary.

[FGasper]

It might be cleaner yet to implement it as some sort of state variable in Test::MockFile. That way the process doesn’t need to change its credentials, and the test credentials are abstracted from the process’s.

[toddr]

Example:

my $file = Test::MockFile->file( $file, $contents, { uid => 0, mode => 0600 } );
$> = 6;
open(my $fh, '<', $file) or die; # Should die because user doesn't have perms.