selftests: tc-testing: preserve list order when removing duplicates

Using set() removes duplicates but breaks ordering.
Test output should be deterministic, so replace with dict.fromkeys().

Signed-off-by: Naveen Anandhan <mr.navi8680@gmail.com>
Link: https://patch.msgid.link/20260222095536.17371-1-mr.navi8680@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>