PR28240: debuginfod client root-safe negative caching

PR28240: debuginfod client root-safe negative caching

Negative cache (000-permission) files were incorrectly treated as
valid cached files for the root user, because root can open even
000-perm files without -EACCES.  Corrected this checking sequence.

Fixed the debuginfod testsuite to run to completion as root or
as an ordinary user, correcting corresponding permission checks:
    stat -c %A $FILE
is right and
    [ -w $FILE]  [ -r $FILE ]
were wrong.

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>