curl: increase test timeouts

curl: increase test timeouts

We often see multiple curl tests fail during ptest runs, the actual test
varies but the output is like this:

  FAIL: 337: protoc!
  There was no content at all in the file log/3/server.input.
  Server glitch? Total curl failure? Returned: 28

Error code 28 is CURLE_OPERATION_TIMEDOUT, so this is almost certainly
due to a loaded machine resulting in the tests running slowly.

It is notable that the test runner explicitly passes --max-time=13 to
curl, so experiment and change this to 600 to see if this solves the
problem.

[ YOCTO #15268 ]

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>