Use 73 bytes der-encoded signatures in weight estimation (#3210)

While 73 bytes der-encoded signatures aren't standard (they're only
possible with high-S signatures), miners could accept such signatures.

Most of our tests use 72 bytes der-encoded signatures because we will
never generate 73 bytes der-encoded signatures ourselves. But it's more
safe to use a 73 bytes signature in our weight estimation because:

- it's what the BOLTs recommend
- it's what other implementations use (e.g. LDK) which means they may
  reject our interactive-tx attempts by thinking we're under-paying
  fees

Author: t-bast

eclair-core/src/main/scala/fr/acinq/eclair/transactions/Transactions.scala

@@ -97,7 +97,9 @@ object Transactions {
   }
 
   sealed trait SegwitV0CommitmentFormat extends CommitmentFormat {
-    override val fundingInputWeight = 384
+    // see https://github.com/lightning/bolts/blob/master/03-transactions.md#appendix-a-expected-weights
+    // funding input weight = 4 * funding_input_size + witness_size = 4 * 41 + 222 = 386
+    override val fundingInputWeight = 386
   }
 
   /**

eclair-core/src/test/scala/fr/acinq/eclair/transactions/TransactionsSpec.scala

@@ -152,8 +152,8 @@ class TransactionsSpec extends AnyFunSuite with Logging {
       case _: SimpleTaprootChannelCommitmentFormat => assert(actual == expected)
       case _: AnchorOutputsCommitmentFormat =>
         // ECDSA signatures are der-encoded, which creates some variability in signature size compared to the baseline.
-        assert(actual <= expected + 3)
-        assert(actual >= expected - 3)
+        assert(actual <= expected + 4)
+        assert(actual >= expected - 4)
     }
   }