Fix deletion tests on Windows 10 1903. (dotnet/corefx#38186)

In the 1903 release of Windows 10 the deletion behavior has changed. Previously, the filename would be reserved until all open handles were closed after being marked for deletion. Now the filename is immediately released.

Commit migrated from dotnet/corefx@a4cf9a4

src/libraries/System.IO.FileSystem/tests/FileStream/ctor_str_fm_fa_fs.delete.cs

@@ -19,9 +19,10 @@ public void FileShareDeleteNew()
             {
                 Assert.True(File.Exists(fileName));
                 File.Delete(fileName);
-                if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows)) // file sharing restriction limitations on Unix
+                if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
                 {
-                    Assert.True(File.Exists(fileName));
+                    // Prior to 1903 Windows would not delete the filename until the last file handle is closed.
+                    Assert.Equal(PlatformDetection.IsWindows10Version1903OrGreater, !File.Exists(fileName));
                 }
             }
 
@@ -56,9 +57,10 @@ public void FileShareDeleteExisting()
             using (FileStream fs = CreateFileStream(fileName, FileMode.Open, FileAccess.ReadWrite, FileShare.Delete))
             {
                 File.Delete(fileName);
-                if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows)) // file sharing restriction limitations on Unix
+                if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
                 {
-                    Assert.True(File.Exists(fileName));
+                    // Prior to 1903 Windows would not delete the filename until the last file handle is closed.
+                    Assert.Equal(PlatformDetection.IsWindows10Version1903OrGreater, !File.Exists(fileName));
                 }
             }
 
@@ -104,16 +106,25 @@ public void FileShareDeleteExistingMultipleClients()
                 {
                     File.Delete(fileName);
                     Assert.Equal(0, fs2.ReadByte());
-                    Assert.True(File.Exists(fileName), $"'{fileName}' should still exist after calling delete with two handles open.");
+
+                    // Prior to 1903 Windows would not delete the filename until the last file handle is closed.
+                    Assert.Equal(PlatformDetection.IsWindows10Version1903OrGreater, !File.Exists(fileName));
                 }
 
                 Assert.Equal(0, fs1.ReadByte());
                 fs1.WriteByte(0xFF);
 
-                // Any attempt to reopen a file in pending-delete state will return Access-denied
-                Assert.Throws<UnauthorizedAccessException>(() => CreateFileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Delete | FileShare.ReadWrite));
-
-                Assert.True(File.Exists(fileName), $"'{fileName}' should still exist after calling delete with inner filestream closed.");
+                if (PlatformDetection.IsWindows10Version1903OrGreater)
+                {
+                    // On 1903 the filename is immediately released after delete is called
+                    Assert.Throws<FileNotFoundException>(() => CreateFileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Delete | FileShare.ReadWrite));
+                }
+                else
+                {
+                    // Any attempt to reopen a file in pending-delete state will return Access-denied
+                    Assert.Throws<UnauthorizedAccessException>(() => CreateFileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Delete | FileShare.ReadWrite));
+                    Assert.True(File.Exists(fileName), $"'{fileName}' should still exist after calling delete with inner filestream closed.");
+                }
             }
 
             Assert.False(File.Exists(fileName));