Extend Azure CLI timeout for tests on arm32v7

[damonbarry]

The end-to-end tests were intermittently (but frequently) failing on Debian 11 arm32v7, which is running on an old Raspberry Pi 3B agent (i.e., slow). We're only running the end-to-end tests on this platform in "minimal" mode, meaning we're just running one basic test--TempSensor--but it was still failing. The problem seems to be that getting a token from the Azure CLI (as with az account get-access-token) can be pretty slow on the old Pi, and can timeout.

I did two things to reduce the chances of failure:

1. In the Microsoft.Azure.Devices.Edge.Test.Common.IotHub class, where the Azure CLI credential is required in four different places (three in the constructor alone), cache and use a single instance of the credential rather than doing new AzureCliCredential() each time. This translates to just a single az account get-access-token call.
2. If we're requesting a new AzureCliCredential on an arm32v7 platform, then increase the timeout for the az account get-access-token call from the default 13 seconds to 60 seconds.

I ran the end-to-end tests a couple times against the change and they pass on all platforms.

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