test: parse_volname

Test to reduce the potential for accidental breakage on regex changes.
And to make sure that all vtype_subdirs are parsed.

Signed-off-by: Alwin Antreich <a.antreich@proxmox.com>

PVE/Storage/Plugin.pm

@@ -457,6 +457,10 @@ my $vtype_subdirs = {
     snippets => 'snippets',
 };
 
+sub get_vtype_subdirs {
+    return $vtype_subdirs;
+}
+
 sub get_subdir {
     my ($class, $scfg, $vtype) = @_;