Add tests for expanding an unpacker buffer

[whoshuu]

No description provided.

[redboltz]

Thank you for sending the PR. The tests are kind of white box tests. They are testing a implementation detail.

If the test is for interface, then

    EXPECT_TRUE(original_capacity + 1u <= u.buffer_capacity());

should be used instead of

    EXPECT_EQ((original_capacity + COUNTER_SIZE) * 2 - COUNTER_SIZE, u.buffer_capacity());

However, I think that tests for implementation are worth to have. If implementation would be changed, tests would also be changed. So I will merge the PR.