selftests: net: cope with slow env in so_txtime.sh test

The mentioned test is failing in slow environments:

  # SO_TXTIME ipv4 clock monotonic
  # ./so_txtime: recv: timeout: Resource temporarily unavailable
  not ok 1 selftests: net: so_txtime.sh # exit=1

Tuning the tolerance in the test binary is error-prone and doomed
to failures is slow-enough environment.

Just resort to suppress any error in such cases. Note to suppress
them we need first to refactor a bit the code moving it to explicit
error handling.

Fixes: af5136f9

 ("selftests/net: SO_TXTIME with ETF and FQ")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/2142d9ed4b5c5aa07dd1b455779625d91b175373.1707730902.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>