Force the line height of the TextStyle used in test_step.dart to be 1.0 for all Text widgets.

The test started failing after https://github.com/flutter/flutter/pull/129724 landed. Assuming that the vertical layout of the test was tight and that the change in the default font's line height is the source of the problem (see https://github.com/flutter/flutter/issues/130732#issuecomment-1639089638).

Fixes https://github.com/flutter/flutter/issues/130732