04dfe8dd45 Add api test to check Arbitrary impls (Shing Him Ng)
678fc71b88 Implement Arbitrary for units types (Shing Him Ng)
Pull request description:
Implement Arbitrary for the rest of the types in `units`. Also moved the implementation in `FeeRate` right before the `tests` module
Closes#3705
ACKs for top commit:
apoelstra:
ACK 04dfe8dd45fae9b55dacfe9eb0d73ea306db14ba; successfully ran local tests
tcharding:
ACK 04dfe8dd45
Tree-SHA512: 156bd26d4de85d484711d476df1d2758805387125209f0307aa786dd1585ff9953dbe41b0864b00ae101419176647e3bde7994ed9257c18307d161463b1c8d2e