Inserting a line of code in a method only when unit-testing

Hello i am unit testing a method which deals some I/O operations.I was wondering if there is some kind of way to pause that method for a specific time , at a specific line , without altering the original code.

Original Method

public async Task<int> SomeMethodAsync()
{
    var x=DoSomething1();
    int value=await DoSomething2Async(); 
    return value;
}

Look at the method above.Is there any way from a TestCase to add a delay or a pause at a specific location in the target method? I would like my method to become something like this Only when unit testing !:

Desired method when unit testing

  public async Task<int> SomeMethodAsync()
    {
         var x=DoSomething1();   //line index=0
    #    await Task.Delay(delay) //delay set in the testcase
         int x=await DoSomething2Async();  //line index=1
         return x;  
    }

Unit Test

[TestCase]
public void TestSomeMethodWithDelay()
{
    var delay=1000;
    int desiredLineIndex=1;
    int result=await SomeMethodAsync();
}

As you can see i would like when testing a method to insert some code before a target line , in my case an await Task.Delay(interval) where interval is set in the TestCase. I do not want to modify/pollute the source code with preprocessor directives for testing.The only other way i could think is to use something like System.Reflection.Emit and rewrite the target method when testing.

Is there some other elegant way?

P.S I just want , when running a particular set of tests to insert a delay between two lines of code without altering the original code.

Update The SomeMethodAsync is from another dll.The code must not be changed ! The DoSomething2Async is an external call.