[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/Common.Helpers.psm1

@@ -0,0 +1,35 @@
+function Get-CommandResult {
+    param (
+        [Parameter(Mandatory=$true)]
+        [string] $Command,
+        [switch] $Multiline
+    )
+    # Bash trick to suppress and show error output because some commands write to stderr (for example, "python --version")
+    $stdout = & bash -c "$Command 2>&1"
+    $exitCode = $LASTEXITCODE
+    return @{
+        Output = If ($Multiline -eq $true) { $stdout } else { [string]$stdout }
+        ExitCode = $exitCode
+    }
+}
+
+function Get-OSName {
+    lsb_release -ds
+}
+
+function Test-IsUbuntu16 {
+    return (lsb_release -rs) -eq "16.04"
+}
+
+function Test-IsUbuntu18 {
+    return (lsb_release -rs) -eq "18.04"
+}
+
+function Test-IsUbuntu20 {
+    return (lsb_release -rs) -eq "20.04"
+}
+
+function Get-ToolsetContent {
+    $toolset = Join-Path $env:INSTALLER_SCRIPT_FOLDER "toolset.json"
+    Get-Content $toolset -Raw | ConvertFrom-Json
+}