A lot of tests reuse $DEV1 veth name for naming dummy device. This causes
problem when tdc is invoked without specifying a test group and tries to
execute all tests. In this case tdc instantiates ns plugin, which creates
veth pair once before running tests. However, if any of the tests that
reuse $DEV1 run before test that depend on ns plugin, it will delete $DEV1
as a part of teardown section:

=====> Test 3b88: Delete ingress qdisc twice                                                                                                                                                             [3770/41080]
-----> prepare stage
ns/SubPlugin.adjust_command
adjust_command:  stage is setup; inserting netns stuff in command [/sbin/ip link add dev v0p1 type dummy || /bin/true] list [['/sbin/ip', 'link', 'add', 'dev', 'v0p1', 'type', 'dummy', '||', '/bin/true']]
adjust_command:  return command [ip netns exec tcut /sbin/ip link add dev v0p1 type dummy || /bin/true]
command "ip netns exec tcut /sbin/ip link add dev v0p1 type dummy || /bin/true"
ns/SubPlugin.adjust_command
adjust_command:  stage is setup; inserting netns stuff in command [/sbin/tc qdisc add dev v0p1 ingress] list [['/sbin/tc', 'qdisc', 'add', 'dev', 'v0p1', 'ingress']]
adjust_command:  return command [ip netns exec tcut /sbin/tc qdisc add dev v0p1 ingress]
command "ip netns exec tcut /sbin/tc qdisc add dev v0p1 ingress"
ns/SubPlugin.adjust_command
adjust_command:  stage is setup; inserting netns stuff in command [/sbin/tc qdisc del dev v0p1 ingress] list [['/sbin/tc', 'qdisc', 'del', 'dev', 'v0p1', 'ingress']]
adjust_command:  return command [ip netns exec tcut /sbin/tc qdisc del dev v0p1 ingress]
command "ip netns exec tcut /sbin/tc qdisc del dev v0p1 ingress"
-----> execute stage
ns/SubPlugin.adjust_command
adjust_command:  stage is execute; inserting netns stuff in command [/sbin/tc qdisc del dev v0p1 ingress] list [['/sbin/tc', 'qdisc', 'del', 'dev', 'v0p1', 'ingress']]
adjust_command:  return command [ip netns exec tcut /sbin/tc qdisc del dev v0p1 ingress]
command "ip netns exec tcut /sbin/tc qdisc del dev v0p1 ingress"
-----> verify stage
ns/SubPlugin.adjust_command
adjust_command:  stage is verify; inserting netns stuff in command [/sbin/tc qdisc show dev v0p1] list [['/sbin/tc', 'qdisc', 'show', 'dev', 'v0p1']]
adjust_command:  return command [ip netns exec tcut /sbin/tc qdisc show dev v0p1]
command "ip netns exec tcut /sbin/tc qdisc show dev v0p1"
-----> teardown stage
ns/SubPlugin.adjust_command
adjust_command:  stage is teardown; inserting netns stuff in command [/sbin/ip link del dev v0p1 type dummy] list [['/sbin/ip', 'link', 'del', 'dev', 'v0p1', 'type', 'dummy']]
adjust_command:  return command [ip netns exec tcut /sbin/ip link del dev v0p1 type dummy]
command "ip netns exec tcut /sbin/ip link del dev v0p1 type dummy"

After this ns-dependent tests will fail because dev doesn't exist:

=====> Test 901f: Add fw filter with prio at 32-bit maxixum
-----> prepare stage
ns/SubPlugin.adjust_command
adjust_command:  stage is setup; inserting netns stuff in command [/sbin/tc qdisc add dev v0p1 ingress] list [['/sbin/tc', 'qdisc', 'add', 'dev', 'v0p1', 'ingress']]
adjust_command:  return command [ip netns exec tcut /sbin/tc qdisc add dev v0p1 ingress]
command "ip netns exec tcut /sbin/tc qdisc add dev v0p1 ingress"

-----> prepare stage *** Could not execute: "$TC qdisc add dev $DEV1 ingress"

-----> prepare stage *** Error message: "Cannot find device "v0p1"
"
returncode 1; expected [0]

-----> prepare stage *** Aborting test run.

<_io.BufferedReader name=3> *** stdout ***

<_io.BufferedReader name=5> *** stderr ***
"-----> prepare stage" did not complete successfully
Exception <class '__main__.PluginMgrTestFail'> ('setup', None, '"-----> prepare stage" did not complete successfully') (caught in test_runner, running test 477 901f Add fw filter with prio at 32-bit maxixum stage
setup)
---------------
traceback
  File "./tdc.py", line 371, in test_runner
    res = run_one_test(pm, args, index, tidx)
  File "./tdc.py", line 272, in run_one_test
    prepare_env(args, pm, 'setup', "-----> prepare stage", tidx["setup"])
  File "./tdc.py", line 247, in prepare_env
    '"{}" did not complete successfully'.format(prefix))
---------------

Fix the issue by introducing standalone $DUMMY config variable and
substitute all usage of $DEV1 in tests that don't depend on ns plugin.

Fixes: 489ce2f4 ("tc-testing: Restore original behaviour for namespaces in tdc")
Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>