tests: Increase P2P group formation timeout to 20 seconds

tests: Increase P2P group formation timeout to 20 seconds

Number of crypto operations seem to take very long time in the valgrind
tests (about five seconds for passphrase to PSK mapping and for public
key generation for M1 and M2 on a virtual server) and this is enough to
push the test runs to hit the timeout frequently even when there is no
real error. Make this less frequent by increasing group formation
timeout from 15 to 20 seconds to avoid issues based on the test scripts
(15 + config time seconds timeout in the protocol may still kick in,
though).

Signed-hostap: Jouni Malinen <j@w1.fi>