selftests: mptcp: tweak simult_flows for debug kernels

The mentioned test measures the transfer run-time to verify
that the user-space program is able to use the full aggregate B/W.

Even on (virtual) link-speed-bound tests, debug kernel can slow
down the transfer enough to cause sporadic test failures.

Instead of unconditionally raising the maximum allowed run-time,
tweak when the running kernel is a debug one, and use some simple/
rough heuristic to guess such scenarios.

Note: this intentionally avoids looking for /boot/config-<version> as
the latter file is not always available in our reference CI
environments.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Co-developed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>