testsuite: exclude unsupported IDN coverage

.github/workflows/stable-testsuite.yml

@@ -66,7 +66,7 @@ jobs:
       run: |
         sudo apt-get update
         sudo apt-get install -y gcc g++ gawk autoconf automake \
-          acl libacl1-dev attr libattr1-dev libidn2-dev liblz4-dev libzstd-dev libxxhash-dev \
+          acl libacl1-dev attr libattr1-dev liblz4-dev libzstd-dev libxxhash-dev \
           python3-cmarkgfm openssl
         echo "/usr/local/bin" >>"$GITHUB_PATH"
     - name: configure

testsuite/skiplist/backport.txt

@@ -28,6 +28,7 @@ partial-protected-regular-retry-policy  # EACCES recovery path (3.5.0, macOS-onl
 contimeout-rsh                          # daemon-over-rsh timeout support (3.5.1)
 daemon-stdin-local-socket               # local-socket inetd detection (3.5.1)
 install-strip                           # configurable install-strip tool (3.5.1)
+idn                                     # IDN hostname support (3.5.1)
 rsync-ssl-type-option                   # flexible rsync-ssl --type placement (3.5.1)
 write-touched-blocks                    # protocol-33 logical-block statistics (3.5.1)