Save stderr from test output, allow for non-zero exit code from tests.

tests/test_visit.expected

@@ -84,6 +84,7 @@ json_c_visit(stop_array)=0
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 flags: 0x0, key: (null), index: -1, value: { "obj1": 123, "obj2": { "subobj1": "aaa", "subobj2": "bbb", "subobj3": [ "elem1", "elem2", true ] }, "obj3": 1.234, "obj4": [ true, false, null ] }
+ERROR: invalid return value from json_c_visit userfunc: 100
 json_c_visit(err_return)=-1
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