GH Actions: add some additional XML validation checks

* Check the docs against the schema from PHPCSDevTools.
* Check the code style of the XSD file.
* Check dev tool config files.

Author: jrfnl

.github/workflows/validate.yml

@@ -42,11 +42,31 @@ jobs:
           pattern: "./src/Standards/*/ruleset.xml"
           xsd-file: "phpcs.xsd"
 
-      # Validate the basic well-formedness of the Documentation XML files.
+      # Validate the Documentation XML files.
       - name: Validate documentation XML
         uses: phpcsstandards/xmllint-validate@v1
         with:
           pattern: "./src/Standards/*/Docs/*/*Standard.xml"
+          xsd-url: "https://phpcsstandards.github.io/PHPCSDevTools/phpcsdocs.xsd"
+
+      # Validate dev tool related XML files.
+      - name: Validate Project PHPCS ruleset against schema
+        uses: phpcsstandards/xmllint-validate@v1
+        with:
+          pattern: "phpcs.xml.dist"
+          xsd-file: "phpcs.xsd"
+
+      - name: "Validate PHPUnit config for use with PHPUnit 8"
+        uses: phpcsstandards/xmllint-validate@v1
+        with:
+          pattern: "phpunit.xml.dist"
+          xsd-file: "vendor/phpunit/phpunit/schema/8.5.xsd"
+
+      - name: "Validate PHPUnit config for use with PHPUnit 9"
+        uses: phpcsstandards/xmllint-validate@v1
+        with:
+          pattern: "phpunit.xml.dist"
+          xsd-file: "vendor/phpunit/phpunit/schema/9.2.xsd"
 
   xml-cs:
     name: 'XML Code style'
@@ -79,7 +99,9 @@ jobs:
       # Check the code-style consistency of the XML files.
       - name: Check XML code style
         run: |
+          diff -B ./phpcs.xsd <(xmllint --format "./phpcs.xsd")
           diff -B ./phpcs.xml.dist <(xmllint --format "./phpcs.xml.dist")
+          diff -B ./phpunit.xml.dist <(xmllint --format "./phpunit.xml.dist")
           diff -B ./src/Standards/Generic/ruleset.xml <(xmllint --format "./src/Standards/Generic/ruleset.xml")
           diff -B ./src/Standards/MySource/ruleset.xml <(xmllint --format "./src/Standards/MySource/ruleset.xml")
           diff -B ./src/Standards/PEAR/ruleset.xml <(xmllint --format "./src/Standards/PEAR/ruleset.xml")