Fix Android tests, get to 100% coverage.

android/src/toga_android/widgets/button.py

@@ -28,15 +28,14 @@ def get_text(self):
         return str(self.native.getText())
 
     def set_text(self, text):
-        self.native.setText(self.interface.text)
+        self.native.setText(text)
 
     def set_enabled(self, value):
         self.native.setEnabled(value)
 
     def set_font(self, font):
-        if font:
-            self.native.setTextSize(TypedValue.COMPLEX_UNIT_SP, font._impl.get_size())
-            self.native.setTypeface(font._impl.get_typeface(), font._impl.get_style())
+        self.native.setTextSize(TypedValue.COMPLEX_UNIT_SP, font._impl.get_size())
+        self.native.setTypeface(font._impl.get_typeface(), font._impl.get_style())
 
     def set_on_press(self, handler):
         # No special handling required

testbed/tests/assertions.py

@@ -7,9 +7,15 @@
 #   * assert_set_get(obj, name, pytest.approx(value)) - for floating point values
 #   * assert_set_get(obj, name, set_value, get_value) - where the two values are different
 def assert_set_get(obj, name, value):
-    """Calls a setter, then asserts that the same value is returned by the getter."""
+    """Calls a setter, then asserts that the same value is returned by the getter.
+
+    :param obj: The object to inspect
+    :param name: The name of the attribute to set and get
+    :param value: The value to set
+    """
     setattr(obj, name, value)
-    assert getattr(obj, name) == value
+    actual = getattr(obj, name)
+    assert actual == value, f"Expected {value!r}, got {actual!r}"
 
 
 def assert_color(actual, expected):