I have an ngFor loop set up like this:
<div *ngFor="let record of this.RecordsProcessed; let i = index">
   <div class="row my-16" data-test='test'_{{i}}>
        <div class="col-4">Id:</div>
        <div class="col-8">{{record?.Id}}</div>
    </div>
</div>

I want to put the index from ngFor on the data-text tag within the html. Is it possible to do something like this within the html?

How to make the index from ngFor part of an html tag value
Try like this:
  <div *ngFor="let record of this.RecordsProcessed; let i = index">
  <div class="row my-16" [attr.data-test]="'test_' + i">
       <div class="col-4">Id:</div>
       <div class="col-8">{{record?.Id}}</div>
   </div>
</div>

[] brackets let angular know that everything inside of "" is typescript code.
