The below code takes the idea from the Azure Quick Start GitHub repo which allows you to specify place holders for ARM template parameters. The Pester test below replaces these place holders so the template can be tested without any user input.

Test-AzureARMTemplateDeployment.ps1

$templatesFolderPath = Get-ChildItem -Path (Split-Path -Parent $MyInvocation.MyCommand.Path).Replace('tests\unit', 'src') | Where-Object { $_.PSIsContainer -eq $true }

Describe "Test Template Againsts Endpoint" -Tags 'unit' {
    foreach($templateFolderPath in $templatesFolderPath)
    {
        Context "Does The Template Deploy Correctly" {
            $hashReturn = @{}

            $ParametersObject = Get-Content -Path (Join-Path $templateFolderPath.FullName 'azuredeploy.parameters.json') | ConvertFrom-Json

            ($ParametersObject | Select-Object -ExpandProperty parameters).PSObject.Properties | ForEach-Object {
                if($_.value.value -eq 'GEN-UNIQUE')
                {
                    $hashReturn.Add($_.name, "a$(((New-Guid).guid -replace '-').Substring(0, 17))")
                }
                elseif($_.value.value -like "GEN-UNIQUE*")
                {
                    $Number = ($_.value.value -split '-')[2]

                    if($Number -gt 32)
                    {
                        Throw 'Number out of bounds'
                    }

                    $hashReturn.Add($_.name, "a$(((New-Guid).guid -replace '-').Substring(0, $Number))")
                }
                elseif ($_.value.value -eq 'GEN-PASSWORD')
                {
                    $hashReturn.Add($_.name, (ConvertTo-SecureString "aA!!$(((New-Guid).guid -replace '-').Substring(0, 9))" -AsPlainText -Force))
                }
                else
                {
                    $hashReturn.Add($_.name, $_.value.value)
                }
            }

            BeforeAll {
                $metaData = Get-Content -path (Join-Path $templateFolderPath.FullName 'metadata.json') | ConvertFrom-Json

                do {
                    $ResourceGroupName = (New-Guid).Guid
                } until (-not (Get-AzureRmResourceGroup -Name $ResourceGroupName -Location ($metaData.location) -ErrorAction SilentlyContinue))

                New-AzureRmResourceGroup -Name $ResourceGroupName -Location ($metaData.location) -Verbose
            }

            AfterAll {
                Remove-AzureRmResourceGroup -Name $ResourceGroupName -Force
            }

            It "Does endpoint validation pass" {
                Test-AzureRmResourceGroupDeployment -ResourceGroupName $ResourceGroupName -TemplateFile (Join-Path $templateFolderPath.FullName 'azuredeploy.json') -TemplateParameterObject $hashReturn -Verbose 2> $templateError
                
                $templateError | Should BeNullOrEmpty
            }
        }
    }
}