docs: augmenting the test plan with X11 clients (#3397)

## What's new?
- Added manual tests for a number of 2.16 regressions (see the [release
page](https://github.com/canonical/mir/releases) for more info)

## Tickets Created
- canonical/ubuntu-frame#184
- canonical/mir-ci#125
- canonical/mir-ci#124
- canonical/mir-ci#126
- canonical/mir-ci#127
- canonical/mir-ci#128
- canonical/mir-ci#129
- canonical/mir-ci#130
- canonical/mir-ci#131

doc/sphinx/how-to/how-to-test-mir-for-a-release.md

@@ -128,8 +128,21 @@ For each empty box in the matrix above, ensure that the following applications c
    gnome-terminal
    ```
 
-## Mir Console Providers
+5. Test that `X11` apps can be started and can receive input:
+    ```sh
+    sudo apt install x11-apps
+    
+    # first,
+    xeyes
+
+    # then
+    xedit
 
+    # finally
+    xcalc
+    ```
+
+## Mir Console Providers
 For each Ubuntu release ensure that the compositor can start with each of the console providers:
 
 |         | 24.04    | 24.10      |