This requires Forms 11 Update 5 (Version 11.0.2311) or later, because prior to that, it was not possible to have a button on the form call a function in the sandboxed Javascript.

I have tested this successfully on version 11.0.2311.50553.

Add some checkbox fields to your form.  Give them CSS Class name of selectAllCheckboxes.

Add a custom HTML element to your form, add this HTML to it: 

<button onclick="markAllCheckboxes()">Mark All Checkboxes</button>
<br><br>
<button onclick="unmarkAllCheckboxes()">Unmark All Checkboxes</button>

Then add this script to the Javascript (JS) section of the form: 

//When the button is clicked to mark all boxes, run this function.
//Loop through each field with the selectAllCheckboxes class, then loop through
//each checkbox on the field to lookup its value.
//Then set all values on the checkbox field.
window.markAllCheckboxes = function () {
  var allCheckboxes = LFForm.findFieldsByClassName("selectAllCheckboxes");
  allCheckboxes.forEach(function(el) {
    var allCheckboxesOptions = el.options;
    var allCheckboxesValues = [];
    allCheckboxesOptions.forEach(function(el) {
      allCheckboxesValues.push(el.value);
    });
    LFForm.setFieldValues(el, {value: allCheckboxesValues});
  });
};

//When the button is clicked to unmark all boxes, run this function.
//Loop through each field with the selectAllCheckboxes class and clear all values.
window.unmarkAllCheckboxes = function () {
  var allCheckboxes = LFForm.findFieldsByClassName("selectAllCheckboxes");
  allCheckboxes.forEach(function(el) {
    LFForm.setFieldValues(el, "");
  });
};

Now you have two buttons that will mark all checkboxes or unmark all checkboxes on the form.