test_exchange
ASSERT_EQ(layer1_err, test_exchange(file2_s1d1, file2_s1d2));
ASSERT_EQ(layer1_err, test_exchange(file2_s1d2, file2_s1d1));
ASSERT_EQ(EXDEV, test_exchange(file2_s1d1, file2_s1d2));
ASSERT_EQ(EXDEV, test_exchange(file2_s1d2, file2_s1d1));
EXPECT_EQ(EACCES, test_exchange(file1_s1d2, file1_s2d3));