add snapshot test for Mimir's GraphQL schema

[boscohyun]

Create Mimir.Tests proejct if not exists

Add snapshot test for Mimir's GraphQL schema

• snapshot test package: Verify, Verify.Xunit

Example:

[Fact]
public async Task VerifySchemaChange()
{
    var schema = await new ServiceCollection()
        .AddGraphQLServer()
        .AddLib9cGraphQLTypes()
        .AddMimirGraphQLTypes()
        .BuildSchemaAsync();
    Verify(schema.ToString());
}