Fix step validation test for <input type="month">

Per https://html.spec.whatwg.org/multipage/#month-state-(type=month):attr-input-step
and https://html.spec.whatwg.org/multipage/#month-state-(type=month):concept-input-value-string-number
and https://html.spec.whatwg.org/multipage/#the-step-attribute ,
an <input type="month"> suffers from a step mismatch
when the difference (in months) between the selected month and January 1970
is not an integral multiple of the step value.

Old broken test:
  <input> validity expectation: Invalid
  step = 2
  value = 2001-March
  "months since Jan 1970" = (2001-1970)*(12 months/year) + (January - March)
    = 31*12 + 2 = 374
  374 % 2 = 0 ➡️️ Valid
  Valid ≠ Invalid
  ❌ Fail!

Corrected test:
  <input> validity expectation: Invalid
  step = 3
  value = 2001-February
  "months since Jan 1970" = (2001-1970)*(12 months/year) + (January - February)
    = 31*12 + 1 = 373
  373 % 3 = 1 ➡️️ Invalid
  Invalid == Invalid
  ✅ Pass.

html/semantics/forms/constraints/form-validation-validity-valid.html

@@ -62,7 +62,8 @@
       testData: [
         {conditions: {max: "2000-01", value: "2001-01"}, expected: false, name: "[target] validity.valid must be false if validity.rangeOverflow is true"},
         {conditions: {min: "2001-01", value: "2000-01"}, expected: false, name: "[target] validity.valid must be false if validity.rangeUnderflow is true"},
-        {conditions: {step: 2 * 1 * 1, value: "2001-03"}, expected: false, name: "[target] validity.valid must be false if validity.stepMismatch is true"},
+        // Step checks that "months since Jan 1970" is evenly divisible by `step`
+        {conditions: {step: 3, value: "2001-02"}, expected: false, name: "[target] validity.valid must be false if validity.stepMismatch is true"},
         {conditions: {required: true, value: ""}, expected: false, name: "[target] validity.valid must be false if validity.valueMissing is true"}
       ]
     },