testfx: `DeploymentItem` is not os-independant in combination with folders

Describe the bug

Hi,

I hope I am at the right repository for this. If not, please point me to the right one. DeploymentItem is not os-independant when using it with folders. The folder name must end with /on linux and \on windows.

Thank you.

Steps To Reproduce

Example files:

TestFiles1/some_file1
TestFiles2/some_file2
MyTests.cs
MyTests.csproj

// MyTests.cs
[TestClass]
public class MyTests
{
    [TestMethod]
    [DeploymentItem(@"TestFiles1/")]
    public void TestFailsOnWindows()
    {
        var fs = File.Open(@"some_file1", FileMode.Open);
    }
    [TestMethod]
    [DeploymentItem(@"TestFiles2\")]
    public void TestFailsOnLinuxMac()
    {
        var fs = File.Open(@"some_file2", FileMode.Open);
    }
}

<!--MyTests.csproj-->
<!--...-->
  <ItemGroup>
    <Content Include="TestFiles1\**\*">
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
    </Content>
    <Content Include="TestFiles2\**\*">
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
    </Content>
  </ItemGroup>

Expected behavior

Using either \ or / on windows, linux or mac should always work.

Actual behavior

Files are not copied to the test directory and the test fails if the wrong format is used.

Additional context

None.