Add test data in integration tests

I'm writing integration tests for my .NET Core app using xUnit. I have various things I want to test, but for brevity I'll use this example. I need to test these things ("document" is just a string, I'm not dealing with files):

• Register new employee
• Log in as employee
• Add a new document for the employee
• Add a comment to the document

Tests are of course supposed to run independently of each other, but I cannot add a comment to a nonexistent document. I need the document to exist to test adding a comment, but the employee must also exist to add the document.

I have test classes written for all entities I want to use, but I don't know how to actually get them in the database for a given test. How can I "seed" the DB for a given test?

TestStartup.cs:

public class TestFixture : IDisposable
{
    readonly TestServer _server;
    public HttpClient _client { get; }

    public TestFixture()
    {
        var builder = new WebHostBuilder()
            .UseContentRoot(@"..\..\..\..\..\src\MyProj.Web")
            .UseEnvironment("IntegrationTestDevelopment")
            .UseStartup<Web.Startup>();

        _server = new TestServer(builder);

        _client = _server.CreateClient();
        _client.BaseAddress = new Uri("http://localhost:2856");
    }

    public void Dispose()
    {
        _client.Dispose();
        _server.Dispose();
    }
}

Relevant part of Web.Startup:

services.AddEntityFrameworkInMemoryDatabase()
    .AddDbContext<MyContext>(options => options.UseInMemoryDatabase());

Example test class:

public class EmployeeTests : IClassFixture<TestFixture>
{
    public HttpClient _client;

    public EmployeeTests(TextFixture fixture)
    {
        _client = fixture._client;
    }

    [Theory]
    [MemberData("ValidRegistrations")]
    public async Task Register_new_employee_succeeds(EmployeeModel model)
    {
        var request = Helpers.CreateRequestMessage("POST", "/api/employees", model);
        var response = await _client.SendAsync(request);
        Assert.Equal(HttpStatusCode.Created, response.StatusCode);
    }
}