ui5-date-picker: adapt and restore the time zone changing test

[unazko]

Hello colleagues,

Bug Description

The test with name "Calendar selection works on different timezones" is disabled via the following PR: #4428
Enabling strict parsing for the ui5-date-picker component formatter by default causes this particular test to fail.
There is a mocked Date.prototype.getDate method in the test sample, which alongside with strict parsing set to true causes the
DateFormatter to not be able to parse the string values from the input fields.

Steps to Reproduce

1. Go to the local test page: http://localhost:8080/test-resources/pages/DatePicker_test_page.html
2. Type "-6" into the ui5-step-input control with label "Set date 0 Timezone":
3. Press the corresponding "Apply" button
4. Grab any ui5-date-picker component element from the DOM and execute in the console:
   $0.getFormat().parse("Jan 1, 2019")

Expected Behavior

A JS date object should be successfully generated by the parse method. Instead the parse method
returns null.

Isolated Example

Use the test page, which is relevant to the particular test: http://localhost:8080/test-resources/pages/DatePicker_test_page.html

Context

• UI5 Web Components version: latest
• OS/Platform: windows
• Browser: Chrome
• Affected component: ui5-date-picker

Best Regards,
Boyan

[a-z-ivanov]

The DateFormat's parse method wasn't expected to call getDate on the already correctly parsed JS Date :( However the result is declared invalid and replaced with null when the day does not match the one from the string. As we do not have a way to mock the JS Date, I suggest to delete the test.