tests: display tail of bash.trace on failure

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
(cherry picked from commit c268e5d11b)
Signed-off-by: John Johansen <john.johansen@canonical.com>

tests/regression/apparmor/task.yaml

@@ -123,6 +123,8 @@ execute: |
       echo "Test execution logs are in the files bash.{log,err,trace} and are collected as artifacts"
       echo "Bash errors are listed below:"
       cat bash.err
+      echo "Tail of the trace is:"
+      tail bash.trace
       exit 1
     else
       for xfail in ${XFAIL:-}; do