Put helper function below where its called

Reading the code is arguably easier if we have seen the call site before
seeing the function, saves having to think what the function does.

bitcoin/src/merkle_tree/block.rs

@@ -509,10 +509,6 @@ mod tests {
     use crate::internal_macros::hex;
     use crate::{Block, Txid};
 
-    /// accepts `pmt_test_$num`
-    #[cfg(feature = "rand-std")]
-    fn pmt_test_from_name(name: &str) { pmt_test(name[9..].parse().unwrap()) }
-
     #[cfg(feature = "rand-std")]
     macro_rules! pmt_tests {
         ($($name:ident),* $(,)?) => {
@@ -541,6 +537,10 @@ mod tests {
         pmt_test_4095
     );
 
+    /// Parses the transaction count out of `name` with form: `pmt_test_$num`.
+    #[cfg(feature = "rand-std")]
+    fn pmt_test_from_name(name: &str) { pmt_test(name[9..].parse().unwrap()) }
+
     #[cfg(feature = "rand-std")]
     fn pmt_test(tx_count: usize) {
         use core::cmp::min;