tests/integration-test.sh: allow overriding OCI_DELTA to test an alternative binary

tests/integration-test.sh currently hardcodes:

OCI_DELTA="$PROJECT_DIR/oci-delta"

always testing a local `make build` output. For downstream QE processes (e.g. RHEL RC baseline testing), it's useful to run this same test suite against an already-built binary instead — for example the exact RPM shipped in a distro compose — to verify that specific build rather than a fresh git build.

Suggest making this overridable, defaulting to current behavior:

OCI_DELTA="${OCI_DELTA:-$PROJECT_DIR/oci-delta}"

Start with tests/integration-test.sh and inspect the OCI_DELTA assignment and how the integration suite invokes the binary. Verify the suite still defaults to the local make build output and also uses an externally supplied OCI_DELTA; done means both execution paths work.

Written by the indexing model from the issue text.