Use "a".repeats() instead of manual implementation

Clippy emits:

  warning: manual implementation of `str::repeat` using iterators

As suggested, use `"a".repeats()`.

Author: tcharding

src/key.rs

@@ -1875,7 +1875,7 @@ mod test {
         assert!(PublicKey::from_str("0218845781f631c48f1c9709e23092067d06837f30aa0cd0544ac887fe91ddd1").is_err());
         assert!(PublicKey::from_str("xx0218845781f631c48f1c9709e23092067d06837f30aa0cd0544ac887fe91ddd1").is_err());
 
-        let long_str: String = core::iter::repeat('a').take(1024 * 1024).collect();
+        let long_str = "a".repeat(1024 * 1024);
         assert!(SecretKey::from_str(&long_str).is_err());
         assert!(PublicKey::from_str(&long_str).is_err());
     }

src/schnorr.rs

@@ -505,7 +505,7 @@ mod tests {
         )
         .is_err());
 
-        let long_str: String = core::iter::repeat('a').take(1024 * 1024).collect();
+        let long_str: String = "a".repeat(1024 * 1024);
         assert!(XOnlyPublicKey::from_str(&long_str).is_err());
     }