On very slow devices, `snap_update_test.py` times out before the snap commands finish

Bug Description

By default, this script times out after 5 minutes (300 seconds). This is apparently not long enough on very slow devices, like this one we have in the lab. The refresh kernel test is marked as passed, but in fact it has timed out. As a result, the file that's expected by the following test (snapd/snap-verify-after-refresh-...-kernel-to-base-rev) is not present, and this test fails.

When looking at the logs, we can see that after 5 minutes, the project-kernel snap is only 8.7% downloaded, and the script times out, issuing a reboot command (see full command).

$ cat snap-refresh-kernel-project-kernel-to-base-rev.log
Refreshing snap project-kernel from revision 203 to 155
(info) (Doing) Monitoring snap "project-kernel" to determine whether extra refresh steps are required
(info) (Doing) Download snap "project-kernel" (155) from channel "20/stable"
(info) (Doing) Monitoring snap "project-kernel" to determine whether extra refresh steps are required
(info) (Doing) Download snap "project-kernel" (155) from channel "20/stable" (0.2%)
(...)
(info) (Doing) Download snap "project-kernel" (155) from channel "20/stable" (8.7%)
(info) (Doing) Monitoring snap "project-kernel" to determine whether extra refresh steps are required

For the following jobs:

snapd/snap-refresh-type-name-to-stable-rev
snapd/snap-revert-type-name-from-stable-rev
snapd/snap-refresh-type-name-to-base-rev
snapd/snap-revert-type-name-from-base-rev

several things to fix:

Make sure the script fails if timeout is reached
Increase default timeout to something compatible with slower devices
(optional) Show time in the logs, to have a better idea of the time needed when debugging

To Reproduce

Run the snap update tests on a very slow device

Environment

UC20
checkbox20 4.2.0-dev114

Relevant log output

No response

Additional context

No response

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