Added common/phpunit.md page and examples.

pages/common/phpunit.md

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+# phpunit
+
+> PHPUnit command-line test runner
+
+- Run tests in the current direcotry. Note: Expects you to have a 'phpunit.xml'
+
+`phpunit`
+
+- Run tests in a specific file.
+
+`phpunit {{path/to/TestFile.php}}`
+
+- Run tests annotated with the given group.
+
+`phpunit --group {{name}}`
+
+- Run tests and generate a coverage report in HTML.
+
+`phpunit --coverage-html {{directory}}`