The class is added to the field wrapper div. Here you can see the HTML generated by ACF and where the class “my-test-class” is added
<div class="acf-field acf-field-text acf-field-5ec2e6904879a my-test-class" data-name="test_field" data-type="text" data-key="field_5ec2e6904879a">
<div class="acf-label">
<label for="acf-field_5ec2e6904879a">test field</label></div>
<div class="acf-input">
<div class="acf-input-wrap"><input type="text" id="acf-field_5ec2e6904879a" name="acf[field_5ec2e6904879a]"/></div></div>
</div>
