Bump MSTest.TestFramework from 3.5.2 to 3.6.4 in /docs/csharp/language-reference/operators/snippets/shared in the dotnet group across 1 directory

Bump MSTest.TestFramework from 3.5.2 to 3.6.4 in /docs/csharp/language-reference/operators/snippets/shared in the dotnet group across 1 directory #2637