public class DbContext1 : DbContext
{
    public DbContext1(string connectionString)
    {
    }
}

public class Program
{
    private static readonly SemaphoreSlim _lock = new SemaphoreSlim(1, 1);

    public async Task Run()
    {
        await _lock.WaitAsync();
        try
        {
            await InsertDbRow();
        }
        finally
        {
            _lock.Release();
        }
    }

    private static async Task InsertDbRow()
    {
        await RetryWrapperAsync(async () =>
        {
            using DbContext1 dbContext = new DbContext1("con str");
            await dbContext.SaveChangesAsync();
        });
    }

    public static async Task RetryWrapperAsync(Func<Task> operation)
    {
        for (int i = 0; i < 3; i++)
        {
            try
            {
                await operation();
                break;
            }
            catch (Exception)
            {
                await Task.Delay(100);
            }
        };
    }
}

[TestClass]
public class ProgramTest
{
    private readonly DbContext1 mockContext1 = Mock.Create<DbContext1>();

    [TestInitialize]
    public void SetUp()
    {
        Mock.Arrange(() => new DbContext1("con str")).Returns(mockContext1);
    }

    [TestMethod]
    public async Task TestMethod()
    {
        // Arrange
        Program program = new Program();

        // Act
        await program.Run(); // <-- at this point the test hangs
    }
}