add "--threads:on" to tests

and be explicit about relative path imports in them

tests/nim.cfg

@@ -0,0 +1,2 @@
+--threads:on
+

tests/test_lexer.nim

@@ -1,6 +1,6 @@
 import
   unittest,
-  ../json_serialization/lexer, utils
+  ../json_serialization/lexer, ./utils
 
 template expectedToken(token: TokKind, additionalTest = true) {.dirty.} =
   lexer.next()

tests/test_serialization.nim

@@ -1,7 +1,7 @@
 import
   strutils, options, unittest,
   serialization/testing/generic_suite,
-  ../json_serialization, utils
+  ../json_serialization, ./utils
 
 type
   Simple = object