fix: show output panel command (#266)

The Show Output Panel requires the name to be UnitTesting:

    "caption": "UnitTesting: Show Output Panel",
    "command": "show_panel", "args": {"panel": "output.UnitTesting"}

unittesting/base.py

@@ -166,7 +166,7 @@ def load_stream(self, package, settings):
         output = settings["output"]
         if not output or output == "<panel>":
             output_panel = OutputPanel(
-                self.window, "exec", file_regex=r'File "([^"]*)", line (\d+)'
+                self.window, "UnitTesting", file_regex=r'File "([^"]*)", line (\d+)'
             )
             output_panel.show()
             return output_panel