Fix integration lock file

Checking in the lock file for the integration tests package since an
update to cargo-rbmt started to use the --locked flag which breaks
on any dependency version changes. This might need to be re-worked to
be more maintainable, I could see it falling out of date. But will give
it a shot first.

Author: nyonson

.gitignore

@@ -1,8 +1,9 @@
 # .gitignore for rust-psbt repository.
 
-# Lock files
-Cargo.lock
+# Ignore library lock file.
+/Cargo.lock
+# But check in lock file for test package infra.
+!/bitcoind-tests/Cargo.lock
 
 # Build artifacts
 target
-