ASTF: Fix syntax error in stop

ASTF: Fix syntax error in stop

When ASTF suites were introduced,
they already edited [0] actions to avoid explicit stop.
Then a refactor [1] introduced the syntax error,
but as stop never run, we didnot see it as a failure.
Finally another refactor [2] caused the stop to be called again,
exposing the syntax error.

As a related issue, pylint script was not updated to check files
in GPL directory until [3].

[0] https://gerrit.fd.io/r/c/csit/+/28205/8/tests/vpp/perf/ip4/2n1l-10ge2p1x710-ethip4udp-ip4base-h1024-p63-s64512-ndrpdr.robot#84
[1] https://gerrit.fd.io/r/c/csit/+/28208/179/GPL/tools/trex/trex_astf_stop.py#102
[2] https://gerrit.fd.io/r/c/csit/+/32030/37/resources/libraries/robot/performance/performance_utils.robot#518
[3] https://gerrit.fd.io/r/c/csit/+/32504

Change-Id: Ibd20b43cb263170d9f17255ff1bcd20f09c42cb5
Signed-off-by: Vratko Polak <vrpolak@cisco.com>