Issue #3311001 by phenaproxima: Document the fixtures used by StagedProjectsValidatorTest

tests/fixtures/StagedProjectsValidatorTest/README.md

+# `StagedProjectsValidatorTest` Fixtures
+
+### new_project_added
+Simulates a scenario where, while updating Drupal core in a site with no non-core dependencies, a new contrib module and
+a new custom module are unexpectedly installed (as runtime and dev dependencies, respectively). Additionally, two new
+non-Drupal packages are installed (again, one as a runtime dependency, the other dev).
+
+**Expectation**: The validator should complain about the new modules; the new non-Drupal packages are ignored.
+
+### no_errors
+Simulates a scenario where, while updating Drupal core in a site with two unpinned contrib dependencies (one runtime and
+one dev), no Drupal packages are updated, but two non-Drupal libraries are removed (again, one a runtime dependency, the
+other dev), two are updated (same arrangement), and two are added (ditto).
+
+**Expectation**: The validator to raise no errors; changes to non-Drupal packages are ignored.
+
+### project_removed
+Simulates a scenario where, while updating Drupal core in a site with no non-core dependencies, an installed contrib
+theme and an installed custom theme are unexpectedly removed (from runtime and dev dependencies, respectively).
+Additionally, two installed non-Drupal packages are removed (again, one from a runtime dependency, the other dev). The 
+existing contrib dependencies' installed versions are unchanged.
+
+**Expectation**: The validator should complain about the removed themes; the removed non-Drupal packages are ignored.
+
+### version_changed
+Simulates a scenario where, while updating Drupal core in a site with two unpinned contrib dependencies (one runtime and
+one dev), the contrib modules are unexpectedly updated, as are two installed non-Drupal packages (again, one a runtime
+dependency, the other dev).
+
+**Expectation**: The validator should complain about the updated modules; the updated non-Drupal packages are ignored.

tests/fixtures/project_staged_validation/no_core_requirements/composer.json

-{
-    "require": {
-        "drupal/core-composer-scaffold": "*",
-        "drupal/core-vendor-hardening": "*",
-        "drupal/core-project-message": "*",
-        "drupal/core-dev": "*",
-        "drupal/core-dev-pinned": "*"
-    },
-    "extra": {
-        "_comment": [
-            "This is an example composer.json that does not require Drupal core.",
-            "@see \\Drupal\\Tests\\automatic_updates\\Kernel\\ReadinessValidation\\CoreComposerValidatorTest"
-        ]
-    }
-}

tests/fixtures/project_staged_validation/no_core_requirements/vendor/composer/installed.json

-{
-    "packages": [
-        {
-            "name": "drupal/core-recommended",
-            "version": "9.8.0"
-        },
-        {
-            "name": "drupal/core",
-            "version": "9.8.0"
-        }
-    ]
-}