Migrate toolcache / toolset tests to Pester (#1222)

* implement toolset tests

* Update TestsHelpers.ps1

* Update InstallHelpers.ps1

images/win/scripts/ImageHelpers/TestsHelpers.ps1

@@ -12,6 +12,11 @@ function Get-CommandResult {
     }
 }
 
+# Gets path to the tool, analogue of 'which tool'
+function Get-WhichTool($tool) {
+    return (Get-Command $tool).Path
+}
+
 # Gets value of environment variable by the name
 function Get-EnvironmentVariable($variable) {
     return [System.Environment]::GetEnvironmentVariable($variable)