How to execute edit mode tests from script. Only subscribes to finished events. Requires that you can reference test assemblies.

ExecuteEditorTests.cs

private static void ExecuteTests()
{
    // Select all types in appdomain
    var types = AppDomain.CurrentDomain.GetAssemblies().SelectMany(a => a.GetTypes()).ToArray();

    // Need to create an empty runner filter
    var filterType = types.First(t => t.Name == "TestRunnerFilter");
    var filter = Activator.CreateInstance(filterType);

    // Create a test launcher
    var launcherType = types.First(t => t.Name == "EditModeLauncher");
    var launcher = Activator.CreateInstance(launcherType, filter, TestPlatform.EditMode);
    
    // Grab the runner from the launcher to subscribe to test events
    var runner = launcherType.GetField("m_EditModeRunner", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(launcher);
    var evt = (UnityEvent<ITestResult>)runner.GetType().GetField("m_TestFinishedEvent", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(runner);
    evt.AddListener(OnTestComplete);
    
    // Finally run all events
    launcherType.GetMethod("Run").Invoke(launcher, null);
}

private static void OnTestComplete(ITestResult result)
{
    // Root level test node in test window
    if(result.Name == Application.productName
       && Equals(result.ResultState, ResultState.Success))
    {
        // All tests succeeded
    }
}

I figured this out so I could integrate running tests into my in-editor build pipeline.

Make sure that if you use an asmdef file for this script that you enable the Test Assemblies references.

If you want to learn more I recommend grabbing a copy of Jetbrains DotPeek and checking out the decompiled source for EditModeRunner.cs in UnityEditor.TestRunner.dll.