selftests: add xfrm policy insertion speed test script

Nothing special, just test how long insertion of x policies takes.
This should ideally show linear insertion speeds.

Do not run this by default, it has little value, but it can be useful to
check for insertion speed chahnges when altering the xfrm policy db
implementation.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>