selftests: tc: set timeout to 15 minutes

When looking for something else in LKFT reports [1], I noticed that the
TC selftest ended with a timeout error:

  not ok 1 selftests: tc-testing: tdc.sh # TIMEOUT 45 seconds

The timeout had been introduced 3 years ago, see the Fixes commit below.

This timeout is only in place when executing the selftests via the
kselftests runner scripts. I guess this is not what most TC devs are
using and nobody noticed the issue before.

The new timeout is set to 15 minutes as suggested by Pedro [2]. It looks
like it is plenty more time than what it takes in "normal" conditions.

Fixes: 852c8cbf ("selftests/kselftest/runner.sh: Add 45 second timeout per test")
Cc: stable@vger.kernel.org
Link: https://qa-reports.linaro.org/lkft/linux-next-master/build/next-20230711/testrun/18267241/suite/kselftest-tc-testing/test/tc-testing_tdc_sh/log [1]
Link: https://lore.kernel.org/netdev/0e061d4a-9a23-9f58-3b35-d8919de332d7@tessares.net/T/ [2]
Suggested-by: Pedro Tammela <pctammela@mojatatu.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Reviewed-by: Zhengchao Shao <shaozhengchao@huawei.com>
Link: https://lore.kernel.org/r/20230713-tc-selftests-lkft-v1-1-1eb4fd3a96e7@tessares.netAcked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/tc-testing/settings

+timeout=900