fix j1939 tests

Author: ebroecker

src/canmatrix/j1939_decoder.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 from builtins import *
-
+import os
 import attr
 
 import canmatrix.formats
@@ -16,7 +16,7 @@
 @attr.s
 class j1939_decoder(object):
     #string = read_binary(__name__.rpartition('.')[0], "j1939.dbc")
-    with open( "j1939.dbc", "rb") as fd:
+    with open(os.path.join(os.path.dirname(__file__), "j1939.dbc"), "rb") as fd:
         string = fd.read()
     j1939_db = canmatrix.formats.loads_flat(
         string, import_type="dbc", dbcImportEncoding="utf8"