test_run.lua: add ignore_zero option to get_vclock

Sometimes we don't care about zero component of the vclock, for example
we want to ignore it when waiting for replica sync, using vclock (don't
care about local spaces).

test_run.lua

@@ -79,8 +79,13 @@ local function wait_lsn(self, waiter, master)
     end
 end
 
-local function get_vclock(self, node)
-    return self:get_param(node, 'vclock')[1]
+local function get_vclock(self, node, opts)
+    local opts = opts or {}
+    local vclock = self:get_param(node, 'vclock')[1]
+    if opts.ignore_zero then
+        vclock[0] = nil
+    end
+    return vclock
 end
 
 local function wait_vclock(self, node, to_vclock)