xdr: Add marshal binary compress test

[2opremio]

I wanted to make sure nothing broke on #4071 and there were no pre-existing tests.

For the record, I run the tests with the prior implementation and I could only see two justified differences:

1. For LedgerEntryTypeOffer:

Expected :[]byte{0x2, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x3}
Actual   :[]byte{0x2, 0x0, 0x1d, 0x4, 0x9a, 0x80, 0xf, 0xda, 0x8f, 0xab, 0xe8, 0xf6, 0x9d, 0x10, 0xdd, 0x8d, 0xda, 0x79, 0x29, 0x5a, 0x14, 0x87, 0xca, 0xe2, 0x3e, 0x43, 0x4e, 0xf5, 0xab, 0x68, 0xec, 0x13, 0x6c, 0xf3, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x3}          	

Which makes sense since we removed the SellerID from the output (which these 33 bytes []{0x0, 0x1d, 0x4, 0x9a, 0x80, 0xf, 0xda, 0x8f, 0xab, 0xe8, 0xf6, 0x9d, 0x10, 0xdd, 0x8d, 0xda, 0x79, 0x29, 0x5a, 0x14, 0x87, 0xca, 0xe2, 0x3e, 0x43, 0x4e, 0xf5, 0xab, 0x68, 0xec, 0x13, 0x6c, 0xf3})

2. For LedgerEntryTypeTrustline:

expected: []byte{0x1, 0x0, 0x1d, 0x4, 0x9a, 0x80, 0xf, 0xda, 0x8f, 0xab, 0xe8, 0xf6, 0x9d, 0x10, 0xdd, 0x8d, 0xda, 0x79, 0x29, 0x5a, 0x14, 0x87, 0xca, 0xe2, 0x3e, 0x43, 0x4e, 0xf5, 0xab, 0x68, 0xec, 0x13, 0x6c, 0xf3, 0x1, 0x45, 0x55, 0x52, 0x0, 0x1d, 0x4, 0x9a, 0x80, 0xf, 0xda, 0x8f, 0xab, 0xe8, 0xf6, 0x9d, 0x10, 0xdd, 0x8d, 0xda, 0x79, 0x29, 0x5a, 0x14, 0x87, 0xca, 0xe2, 0x3e, 0x43, 0x4e, 0xf5, 0xab, 0x68, 0xec, 0x13, 0x6c, 0xf3}
actual  : []byte{0x1, 0x0, 0x1d, 0x4, 0x9a, 0x80, 0xf, 0xda, 0x8f, 0xab, 0xe8, 0xf6, 0x9d, 0x10, 0xdd, 0x8d, 0xda, 0x79, 0x29, 0x5a, 0x14, 0x87, 0xca, 0xe2, 0x3e, 0x43, 0x4e, 0xf5, 0xab, 0x68, 0xec, 0x13, 0x6c, 0xf3, 0x1, 0x45, 0x55, 0x52, 0x0, 0x0, 0x0, 0x0, 0x1d, 0x4, 0x9a, 0x80, 0xf, 0xda, 0x8f, 0xab, 0xe8, 0xf6, 0x9d, 0x10, 0xdd, 0x8d, 0xda, 0x79, 0x29, 0x5a, 0x14, 0x87, 0xca, 0xe2, 0x3e, 0x43, 0x4e, 0xf5, 0xab, 0x68, 0xec, 0x13, 0x6c, 0xf3}

There are three extra zeros {0x0, 0x0, 0x0}, which also makes sense because. For some reason, we weren't compressing assets issuers:

go/xdr/asset.go

Line 290 in d9d4986

issuer, err = a.AlphaNum4.Issuer.MarshalBinary()

(note that it isn't a binary compression call, but a normal binary call)