[Ubuntu] Implement first Pester tests (#2270)

* implement first pester tests

* add comment for azcopy test

* remove extra importing and old function

* resolve comments

* fix typo

images/linux/scripts/helpers/Tests.Helpers.psm1

@@ -0,0 +1,67 @@
+Import-Module "$PSScriptRoot/../helpers/Common.Helpers.psm1" -DisableNameChecking
+
+# Validates that tool is installed and in PATH
+function Validate-ToolExist($tool) {
+    Get-Command $tool -ErrorAction SilentlyContinue | Should -BeTrue
+}
+
+function Invoke-PesterTests {
+    Param(
+        [Parameter(Mandatory)][string] $TestFile,
+        [string] $TestName
+    )
+
+    $testPath = "/imagegeneration/tests/${TestFile}.Tests.ps1"
+    if (-not (Test-Path $testPath)) {
+        throw "Unable to find test file '$TestFile' on '$testPath'."
+    }
+
+    $configuration = [PesterConfiguration] @{
+        Run = @{ Path = $testPath; PassThru = $true }
+        Output = @{ Verbosity = "Detailed" }
+    }
+    if ($TestName) {
+        $configuration.Filter.FullName = $TestName
+    }
+
+    # Switch ErrorActionPreference to Stop temporary to make sure that tests will fail on silent errors too
+    $backupErrorActionPreference = $ErrorActionPreference
+    $ErrorActionPreference = "Stop"
+    $results = Invoke-Pester -Configuration $configuration
+    $ErrorActionPreference = $backupErrorActionPreference
+
+    # Fail in case if no tests are run
+    if (-not ($results -and ($results.FailedCount -eq 0) -and ($results.PassedCount -gt 0))) {
+        $results
+        throw "Test run has failed"
+    }
+}
+
+function ShouldReturnZeroExitCode {
+    Param(
+        [String] $ActualValue,
+        [switch] $Negate,
+        [string] $Because # This parameter is unused but we need it to match Pester asserts signature
+    )
+
+    $result = Get-CommandResult $ActualValue
+
+    [bool]$succeeded = $result.ExitCode -eq 0
+    if ($Negate) { $succeeded = -not $succeeded }
+
+    if (-not $succeeded)
+    {
+        $commandOutputIndent = " " * 4
+        $commandOutput = ($result.Output | ForEach-Object { "${commandOutputIndent}${_}" }) -join "`n"
+        $failureMessage = "Command '${ActualValue}' has finished with exit code`n${commandOutput}"
+    }
+
+    return [PSCustomObject] @{
+        Succeeded      = $succeeded
+        FailureMessage = $failureMessage
+    }
+}
+
+If (Get-Command -Name Add-AssertionOperator -ErrorAction SilentlyContinue) {
+    Add-AssertionOperator -Name ReturnZeroExitCode -InternalName ShouldReturnZeroExitCode -Test ${function:ShouldReturnZeroExitCode}
+}