add xxd tool for binary dumps from compat tests

xxd is used by the compat test scripts to output binary dumps
of mismatching outputs for manual inspection later.

(So far the absence of this tool went by unnoticed, because the
tests had always passed)

Author: wnienhaus

.github/workflows/run_tests.yaml

@@ -13,7 +13,7 @@ jobs:
     - name: Install dependencies
       run: |
         sudo apt-get update
-        sudo apt-get install -y git build-essential libffi-dev pkg-config python3 bison flex
+        sudo apt-get install -y git build-essential libffi-dev pkg-config python3 bison flex xxd
 
     - name: Record version
       run: |