smoketest: T9048: harden DHCP client process checks against CI timing races

Interface smoketests fail intermittently in CI on DHCP-related
assertions (test_interfaces_cli job pass rate 42-55% over the last 30
workflow runs). Three timing hazards in the interface test base class:

* process_named_running() was polled with a 10 second window at all
  dhclient/dhcp6c call sites - too short on a loaded runner. Raise to a
  shared PROCESS_WAIT_TIMEOUT of 60 seconds; the poll returns as soon
  as the process appears, so this only delays the failure path.

* /proc/<pid>/cmdline was read unguarded after PID discovery. dhclient
  re-executes itself while daemonizing, so the discovered PID can be
  gone by the time /proc is read - read_file() then raises and the test
  errors out instead of failing cleanly. New get_process_cmdline()
  helper re-resolves the PID once when the read fails.

* tearDown() asserted daemon absence immediately after the config was
  removed, reporting daemons still in their shutdown path as leaks.
  Use the existing wait_for_result() shim helper with the same 60s
  bound to grant a grace period before declaring a leak; the poll
  returns on first observation of a clean state, so passing runs pay
  no extra wall clock.

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