USB: usbtest: add a timeout for scatter-gather tests
In usbtest, tests 5 - 8 use the scatter-gather library in usbcore
without any sort of timeout. If there's a problem in the gadget or
host controller being tested, the test can hang.
This patch adds a 10-second timeout to the tests, so that they will
fail gracefully with an ETIMEDOUT error instead of hanging.