Fix search of krb5 plugin directory

The naive find command currently used to find the krb5 plugin directory
can fail, even if it finds a valid path, if it encouters non-readable
directories (missing the rx) along the way.

Instead, we now prune non-readable directories from the find results
via an additional find clause.

Fixes #4

[sross@redhat.com: added additional commit description]

k5test/_utils.py

@@ -97,8 +97,9 @@ def _decide_plugin_dir(dirs):
 
 def _find_plugin_dirs_installed(search_path):
     try:
-        options_raw = get_output('find %s/ -type d '
-                                 '-path "*/krb5/plugins"' % search_path,
+        options_raw = get_output('find %s/ -type d \( ! -executable -o ! -readable \) '
+                                 '-prune -o '
+                                 '-type d -path "*/krb5/plugins" -print' % search_path,
                                  stderr=subprocess.STDOUT)
     except subprocess.CalledProcessError:
         options_raw = None