[Az.Functions] Add test coverage for Go runtime on Flex Consumption before GA

### Description

PR #29816 added support for creating **Go** function apps on Flex Consumption via `New-AzFunctionApp -Runtime Go`, along with tab-completion and `Get-AzFunctionAppFlexConsumptionRuntime` support. The Go runtime on Flex Consumption is currently in preview. We should add coverage before the runtime reaches GA to prevent regressions.

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