MockBlob takes unnecessary generic.

[bkolad]

Summary

MockBlob takes unnecessary generic:

pub struct MockBlob<A> {
    address: A,
    hash: [u8; 32],
    data: CountedBufReader<Bytes>,
}

The address in MockBlob should be set to MockAddress, unfortunately, some of our tests use CelestiaAddress.
The task is:

1. Fix tests to use MockAddress instead of CelestiaAddress
2. Remove generic A from mock blob.