tests/boot-serial-test: Bump timeout to 6 minutes

tests/boot-serial-test: Bump timeout to 6 minutes

On a SPARC host that I'm using as a build test machine, the
boot-serial-test for the SPARC guest machines takes about 65
seconds to execute. This means that it hits the current
60 second timer on these tests. Push the timeout up so
that it doesn't trigger spuriously on slow hosts like this one.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Acked-by: Mark Cave-Ayland <[email protected]>
Message-id: 20180817161404[email protected]