Initial GUI tests

The GUI is rather useless when the app is being run in the test environment, as there are no printers connected (there is no USB backend at all).
This simple test verifies the most basic existence of selected widget, and their content.
We will need to figure out how to test the app with a state that resembles actual printer. We might need to introduce test mode, in which all supported printers are shown in the device selector, so we can test various features for each.

pyproject.toml

@@ -41,7 +41,8 @@ requires-python = ">=3.8,<4"
 [project.optional-dependencies]
 test = [
     "pytest-cov",
-    "pytest-image-diff"
+    "pytest-image-diff",
+    "pytest-qt",
 ]
 
 [project.urls]

src/labelle/gui/tests/test_gui.py

@@ -0,0 +1,25 @@
+from labelle.gui.gui import LabelleWindow
+from labelle.gui.q_label_widgets import (
+    TextDymoLabelWidget,
+)
+
+
+def test_main_window(qtbot):
+    widget = LabelleWindow()
+    qtbot.addWidget(widget)
+
+    assert not widget._actions.isEnabled()
+    assert widget._device_selector.isEnabled()
+    assert not widget._label_list.isEnabled()
+    assert not widget._render_widget.isEnabled()
+    assert not widget._render.isEnabled()
+    assert not widget._settings_toolbar.isEnabled()
+    assert (
+        widget._device_selector._error_label.text()
+        == "Failed scanning devices: No backend available"
+    )
+    assert not widget._actions._print_button.isEnabled()
+    assert widget._label_list.count() == 1
+    item = widget._label_list.itemWidget(widget._label_list.item(0))
+    assert isinstance(item, TextDymoLabelWidget)
+    assert item.label.toPlainText() == "text"