selftests: mptcp: more stable diag tests

selftests: mptcp: more stable diag tests

BugLink: https://bugs.launchpad.net/bugs/1986728
commit 42fb6cddec3b306c9f6ef136b6438e0de1836431 upstream.

The mentioned test-case still use an hard-coded-len sleep to
wait for a relative large number of connection to be established.

On very slow VM and with debug build such timeout could be exceeded,
causing failures in our CI.

Address the issue polling for the expected condition several times,
up to an unreasonable high amount of time. On reasonably fast system
the self-tests will be faster then before, on very slow one we will
still catch the correct condition.

Fixes: df62f2ec3df6 ("selftests/mptcp: add diag interface tests")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>