bls: debugging utils and fake sig fix

There was a discrepancy between the is_empty fields of fake signatures during testing, so I've added
a small hack to set the is_empty field of a fake signature based on the byte content.
Alternatively, we could just make it so that any fake signature is defined to be equal to any other.

eth2/utils/bls/src/signature.rs

@@ -101,6 +101,12 @@ impl Signature {
     pub fn is_empty(&self) -> bool {
         self.is_empty
     }
+
+    /// Display a signature as a hex string of its bytes.
+    #[cfg(test)]
+    pub fn as_hex_string(&self) -> String {
+        hex_encode(self.as_bytes())
+    }
 }
 
 impl_ssz!(Signature, BLS_SIG_BYTE_SIZE, "Signature");