autopkgtest: don't fail on testbed failures

autopkgtest is flaky and sometimes fails even if the code is completely
fine. When it crashes like this, it exits with code 16.

With this change
all CI runs that exit with code 16 won't be considered failures, but
will just report a warning.

For more info, see `man autopkgtest`

.github/workflows/autopkgtest.yaml

@@ -99,10 +99,21 @@ jobs:
     # Currently this sometimes fails on i386 with
     #   <VirtSubproc>: failure: timed out waiting for 'login prompt on serial
     #     console'
-    # and on arm64 and armhf with
+    # on arm64 and armhf with
     #   host login: <VirtSubproc>: failure: The VM does not start a root shell
     #     on ttyS1 or hvc1 already. The only other supported login mechanism is
     #     through --user and -*** the guest ttyS0
+    # on almost all architectures with
+    #   ERROR: testbed failure: sent `auxverb_debug_fail', got `timeout',
+    #     expected `ok...'
 
     - name: Run autopkgtest
-      run: autopkgtest ../pistache_*.dsc -- autopkgtest-virt-qemu --dpkg-architecture=${{ matrix.arch }} --timeout-reboot=120 --ram-size=3072 $HOME/autopkgtest-testing.img
+      run: |
+        exit_code=0
+        autopkgtest ../pistache_*.dsc -- autopkgtest-virt-qemu --dpkg-architecture=${{ matrix.arch }} --timeout-reboot=120 --ram-size=3072 $HOME/autopkgtest-testing.img || exit_code=$?
+        if [ $exit_code -eq 16 ]
+        then
+          echo "::warning::autopkgtest failed, but it's not your fault"
+        else
+          exit $exit_code
+        fi