📝 Add information about terminal width for testing output

docs/tutorial/testing.md

@@ -81,6 +81,36 @@ If you need a refresher about what is "standard output" and "standard error" che
 
 ///
 
+#### Checking rich output
+
+Rich attempts to adjust the formatting to the connected terminal's size, wrapping or shortening content to make it fit.
+However, in many test environments, there is no terminal attached to the process, and therefore Rich will fallback to a standard terminal width of 80 characters.
+
+In some cases this might not be sufficient to test the output of a command.
+It is possible to override the width of a Rich `Console` object using the environment variable `COLUMNS`.
+This applies to all instances of a console created in the application.
+
+/// tip
+
+To avoid polluting the environment outside of the current test run, use monkeypatch to set the environment variable:
+
+```Python
+@pytest.fixture()
+def console_width(monkeypatch):
+    monkeypatch.setenv("COLUMNS", "200")
+```
+
+///
+
+/// tip
+
+Typer also creates such an instance, which is used for output produced by Typer, such as the help display.
+If you only want to modify the output width of this internal usage, you can set the environment variable `TERMINAL_WIDTH`.
+
+It is necessary to set `TERMINAL_WIDTH` *before* the Typer app is imported.
+
+///
+
 ### Call `pytest`
 
 Then you can call `pytest` in your directory and it will run your tests: