Spin Button Date Picker: Fix unreliable date math test (pull #1639)

* Fix unreliable date math bug
* fix: Down arrow test

When you get towards the end of the month and try to add a month, Javascript Date objects start to do unexpected things. This is causing failures in the CI for all regression tests run on November 30.

This is the issue:

```javascript
let date = new Date('10/31/2020'); 
date.setMonth(date.getMonth + 1);
```
The second line increases the date by one month, resulting in date being December 1st, 2020, because there is no November 31st.

The fix is to put the day of the month to the 1st using `date.setDate(1)` before doing any math, because all months have the 1st of the month.

Co-authored-by: Nick Schonning <nschonni@gmail.com>

test/tests/spinbutton_datepicker.js

@@ -370,17 +370,23 @@ ariaTest(
   exampleFile,
   'spinbutton-down-arrow',
   async (t) => {
-    let control = parseInt(ex.dayNow);
-    let daysInMonth = parseInt(ex.dayMax);
+    let control = 31;
+
+    // Set to December for a 31 day month
+    let monthSpinner = await t.context.session.findElement(
+      By.css(ex.monthSelector)
+    );
+    await monthSpinner.sendKeys(Key.END);
 
     // Send down arrow to day date spinner
     let daySpinner = await t.context.session.findElement(
       By.css(ex.daySelector)
     );
-    await daySpinner.sendKeys(Key.ARROW_DOWN);
 
-    // Subtract a day to the control
-    control = (control - 1) % daysInMonth;
+    // Set to first of month
+    await daySpinner.sendKeys(Key.HOME);
+
+    await daySpinner.sendKeys(Key.ARROW_DOWN);
 
     t.is(
       parseInt(await daySpinner.getText()),
@@ -395,7 +401,7 @@ ariaTest(
     }
 
     // Subtract 30 days to the control
-    control = daysInMonth + ((control - 30) % daysInMonth);
+    control -= 30;
 
     t.is(
       parseInt(await daySpinner.getText()),
@@ -408,6 +414,7 @@ ariaTest(
 
 ariaTest('up arrow on month', exampleFile, 'spinbutton-up-arrow', async (t) => {
   let date = new Date();
+  date.setDate(1); // This is necessary to do the correct date math for months.
 
   let monthSpinner = await t.context.session.findElement(
     By.css(ex.monthSelector)
@@ -417,7 +424,6 @@ ariaTest('up arrow on month', exampleFile, 'spinbutton-up-arrow', async (t) => {
   for (let i = 1; i <= 12; i++) {
     await monthSpinner.sendKeys(Key.ARROW_UP);
     const index = new Date(date.setMonth(date.getMonth() + 1)).getMonth();
-    console.log(index);
     t.is(
       await monthSpinner.getText(),
       valuesMonth[index],