tests: Increase presubmit (Kokoro) timeout to 6 hours
tests: Increase presubmit (Kokoro) timeout to 6 hours

Kokoro is now frequnetly timing out. This will increase the timeout from
5 hours to 6 hours.

Change-Id: I2124567142358ab183d962fcbd73ee9ea4e809a3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40455
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Reviewed-by: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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