Merged PR 112: Add check for VS150COMNTOOLS when looking for vstest.console.exe

This is so Win2D can be built on machines that don't have VS2015 installed.

Win2D.proj

@@ -328,6 +328,7 @@
     <_VsTestPathUnderCommonTools>..\IDE\CommonExtensions\Microsoft\TestWindow\vstest.console.exe</_VsTestPathUnderCommonTools>
     <_VsTest Condition="Exists('$(VS120COMNTOOLS)$(_VsTestPathUnderCommonTools)')">$(VS120COMNTOOLS)$(_VsTestPathUnderCommonTools)</_VsTest>
     <_VsTest Condition="Exists('$(VS140COMNTOOLS)$(_VsTestPathUnderCommonTools)')">$(VS140COMNTOOLS)$(_VsTestPathUnderCommonTools)</_VsTest>
+    <_VsTest Condition="Exists('$(VS150COMNTOOLS)$(_VsTestPathUnderCommonTools)')">$(VS150COMNTOOLS)$(_VsTestPathUnderCommonTools)</_VsTest>
     <VsTest>"$([System.IO.Path]::GetFullPath('$(_VsTest)'))"</VsTest>
   </PropertyGroup>