{CI} Increase TestExtensionsLoading's timeout

[jiasli]

Description

With the increasing number of Azure CLI extensions, TestExtensionsLoading now fails with

https://dev.azure.com/azure-sdk/public/_build/results?buildId=1475520&view=logs&j=dec4b9a0-9c0f-52f1-3e08-6456fce2bf62

##[error]The job running on agent [Azure Pipelines](https://dev.azure.com/azure-sdk/29ec6040-b234-4e31-b139-33dc4287b756/_settings/agentqueues?poolId=&queueId=57) 96 ran longer than the maximum time of 20 minutes. For more information, see https://go.microsoft.com/fwlink/?linkid=2077134
Pool: Azure Pipelines
Image: ubuntu-20.04
Agent: Azure Pipelines 96
Started: Today at 1:13 PM
Duration: 20m 7s

This PR increases the timeout threshold.

[jiasli]

CI failed:

https://dev.azure.com/azure-sdk/public/_build/results?buildId=1475575&view=logs&j=7621d84f-e7df-5fb0-9a4f-5fbdcb3421ee&t=bbcd1db7-629e-5645-8ed8-a18b90e0a11a&l=1217

Verifying extension: db-up
WARNING: Unable to load extension 'alias: cannot import name 'soft_unicode' from 'markupsafe' (/opt/az/azcliextensions/alias/markupsafe/__init__.py)'. Use --debug for more information.
WARNING: This package of the Batch Extensions module supports Azure Batch up to version 7.1. The current version 12.0.0 has not been tested for compatibility.
WARNING: This package of the Batch Extensions module supports Azure Batch Management up to version 6.1. The current version 16.1.0 has not been tested for compatibility.
WARNING: This package of the Batch Extensions module supports Azure Batch Extensions up to version 6.1. The current version 8.0.0 has not been tested for compatibility.
WARNING: Failed to delete '/opt/az/azcliextensions/db-up': [Errno 2] No such file or directory: '/opt/az/azcliextensions/db-up'. Retrying ...
WARNING: Failed to delete '/opt/az/azcliextensions/db-up': [Errno 2] No such file or directory: '/opt/az/azcliextensions/db-up'. Retrying ...
WARNING: Failed to delete '/opt/az/azcliextensions/db-up': [Errno 2] No such file or directory: '/opt/az/azcliextensions/db-up'. Retrying ...
WARNING: Failed to delete '/opt/az/azcliextensions/db-up': [Errno 2] No such file or directory: '/opt/az/azcliextensions/db-up'. You may try to delete it manually.
ERROR: An error occurred. Pip failed with status code 1. Use --debug for more information.
##[error]Bash exited with code '1'.

because db-up fails to install on Python 3.10: Azure/azure-cli-extensions#4608

[jiasli]

dp-up extension has been fixed by Azure/azure-cli-extensions#4610.