update markdown tests (#7838)

update markdown tests

  - explicitly use gulp 4.0 beta because 3.9 depends on a vulnerable package

test/tools/Modules/HelpersCommon/HelpersCommon.psd1

@@ -23,6 +23,7 @@ FunctionsToExport = @(
         'Get-RandomFileName'
         'Send-VstsLogFile'
         'Set-TesthookResult'
+        'Start-NativeExecution'
         'Test-IsElevated'
         'Test-IsRoot'
         'Test-IsVstsLinux'

test/tools/Modules/HelpersCommon/HelpersCommon.psm1

@@ -266,3 +266,50 @@ function Test-IsVstsWindows
 {
     return ($env:TF_BUILD -and $IsWindows)
 }
+
+# this function wraps native command Execution
+# for more information, read https://mnaoumov.wordpress.com/2015/01/11/execution-of-external-commands-in-powershell-done-right/
+function Start-NativeExecution
+{
+    param(
+        [scriptblock]$sb,
+        [switch]$IgnoreExitcode,
+        [switch]$VerboseOutputOnError
+    )
+    $backupEAP = $script:ErrorActionPreference
+    $script:ErrorActionPreference = "Continue"
+    try {
+        if($VerboseOutputOnError.IsPresent)
+        {
+            $output = & $sb 2>&1
+        }
+        else
+        {
+            & $sb
+        }
+
+        # note, if $sb doesn't have a native invocation, $LASTEXITCODE will
+        # point to the obsolete value
+        if ($LASTEXITCODE -ne 0 -and -not $IgnoreExitcode) {
+            if($VerboseOutputOnError.IsPresent -and $output)
+            {
+                $output | Out-String | Write-Verbose -Verbose
+            }
+
+            # Get caller location for easier debugging
+            $caller = Get-PSCallStack -ErrorAction SilentlyContinue
+            if($caller)
+            {
+                $callerLocationParts = $caller[1].Location -split ":\s*line\s*"
+                $callerFile = $callerLocationParts[0]
+                $callerLine = $callerLocationParts[1]
+
+                $errorMessage = "Execution of {$sb} by ${callerFile}: line $callerLine failed with exit code $LASTEXITCODE"
+                throw $errorMessage
+            }
+            throw "Execution of {$sb} failed with exit code $LASTEXITCODE"
+        }
+    } finally {
+        $script:ErrorActionPreference = $backupEAP
+    }
+}