This test verifies that MyWidget displays a given title and message.
It is titled accordingly, and it will be populated in the next section.

4. Build the widget using the WidgetTester

Next, build MyWidget inside the test environment by using the
pumpWidget() method provided by WidgetTester.
The pumpWidget method builds and renders the provided widget.

Create a MyWidget instance that displays “T” as the title
and “M” as the message.

Notes about the pump() methods

After the initial call to pumpWidget(), the WidgetTester provides
additional ways to rebuild the same widget. This is useful if you’re
working with a StatefulWidget or animations.

For example, tapping a button calls setState(), but Flutter won’t
automatically rebuild your widget in the test environment.
Use one of the following methods to ask Flutter to rebuild the widget.

tester.pump(Duration duration)