tests: fix function to test for docker OS due to change to use linuxkit for mac (PowerShell#5843)

* Also, fix syntax issues to allow to work with released Pester

Author: TravisEz13

docker/tests/container.tests.ps1

@@ -57,8 +57,7 @@ Describe "Linux Containers run PowerShell" -Tags 'Behavior', 'Linux' {
         # prune unused volumes
         $null=Invoke-Docker -Command 'volume', 'prune' -Params '--force' -SuppressHostOutput
     }
-    BeforeEach
-    {
+    BeforeEach {
         Remove-Item $testContext.resolvedXmlPath -ErrorAction SilentlyContinue
         Remove-Item $testContext.resolvedLogPath -ErrorAction SilentlyContinue
     }
@@ -82,8 +81,7 @@ Describe "Windows Containers run PowerShell" -Tags 'Behavior', 'Windows' {
     BeforeAll{
         $testContext = Get-TestContext -type Windows
     }
-    BeforeEach
-    {
+    BeforeEach {
         Remove-Item $testContext.resolvedXmlPath -ErrorAction SilentlyContinue
         Remove-Item $testContext.resolvedLogPath -ErrorAction SilentlyContinue
     }

docker/tests/containerTestCommon.psm1

@@ -110,7 +110,27 @@ function Test-SkipWindows
 
 function Test-SkipLinux
 {
-    return !((Get-DockerEngineOs) -like 'Alpine Linux*')
+    $os = Get-DockerEngineOs
+
+    switch -wildcard ($os)
+    {
+        '*Linux*' {
+            return $false
+        }
+        '*Mac' {
+            return $false
+        }
+        # Docker for Windows means we are running the linux kernel
+        'Docker for Windows' {
+            return $false
+        }
+        'Windows*' {
+            return $true
+        }
+        default {
+            throw "Unknow docker os '$os'"
+        }
+    }
 }
 
 function Get-TestContext