Merge bitcoindevkit/bdk#614: Avoid using immature coinbase inputs

e85aa247cb Avoid using immature coinbase inputs (Daniela Brozzoni)
0e0d5a0e95 populate_test_db accepts a `coinbase` param (Daniela Brozzoni)

Pull request description:

  ### Description

  With this PR we start considering how many confirmations a coinbase has. If it's not mature yet, we don't use it for building transactions.
  Fixes #413

  ### Notes to the reviewers

  This PR is based on #611, review that one before reviewing this 😄

  007c5a78335a3e9f6c9c28a077793c2ba34bbb4e adds a coinbase parameter to `populate_test_db`, to specify if you want the db to be populated with immature coins. This is useful for `test_spend_coinbase`, but that's probably going to be the only use case.
  I don't think it's a big deal to have a test function take an almost_always_useless parameter - it's not an exposed API, anyways. But, if you can come up with a different way of implementing `test_spend_coinbase` that doesn't require 007c5a78335a3e9f6c9c28a077793c2ba34bbb4e, even better! I looked for it for a while, but other than duplicating the whole `populate_test_db` code, which made the test way harder to comprehend, I didn't find any other way.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [x] I've added tests to reproduce the issue which are now passing
  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  afilini:
    ACK e85aa24

Tree-SHA512: 30f470c33f9ffe928500a58f821f8ce445c653766459465eb005031ac523c6f143856fc9ca68a8e1f23a485c6543a9565bd889f9557c92bf5322e81291212a5f

src/wallet/tx_builder.rs

@@ -547,10 +547,15 @@ impl<'a, D: BatchDatabase, Cs: CoinSelectionAlgorithm<D>, Ctx: TxBuilderContext>
 
     /// Set the current blockchain height.
     ///
-    /// This will be used to set the nLockTime for preventing fee sniping. If the current height is
-    /// not provided, the last sync height will be used instead.
-    ///
+    /// This will be used to:
+    /// 1. Set the nLockTime for preventing fee sniping.
     /// **Note**: This will be ignored if you manually specify a nlocktime using [`TxBuilder::nlocktime`].
+    /// 2. Decide whether coinbase outputs are mature or not. If the coinbase outputs are not
+    ///    mature at `current_height`, we ignore them in the coin selection.
+    ///    If you want to create a transaction that spends immature coinbase inputs, manually
+    ///    add them using [`TxBuilder::add_utxos`].
+    ///
+    /// In both cases, if you don't provide a current height, we use the last sync height.
     pub fn set_current_height(&mut self, height: u32) -> &mut Self {
         self.params.current_height = Some(height);
         self