Fix error handling in _client.lua

Previously we got the following unhelpful error message:

tests/_client.lua:98: bad argument #2 to 'assert' (string expected, got
userdata)

Signed-off-by: Uli Schlachter <psychon@znc.in>

tests/_client.lua

@@ -94,7 +94,7 @@ return function(class, title, sn_rules, callback)
     local snid = (sn_rules or callback) and get_snid(sn_rules, callback) or ""
     local data = class .. "\n" .. title .. "\n" .. snid .. "\n"
     local success, msg = pipe:write_all(data)
-    assert(success, msg)
+    assert(success, tostring(msg))
 
     return snid
 end