Remove strict attribute in the XML configuration

Let PHPUnit be quiet

    PHPUnit 7.5.1 by Sebastian Bergmann and contributors.

      Warning - The configuration file did not pass validation!
      The following problems have been detected:

      Line 8:
      - Element 'phpunit', attribute 'strict': The attribute 'strict' is not allowed.

      Test results may not be as expected.

See sebastianbergmann/phpunit@78c8865

tests/phpunit.xml

@@ -1,10 +1,14 @@
 <phpunit
   colors="true"
+  beStrictAboutCoversAnnotation="true"
+  beStrictAboutOutputDuringTests="true"
+  beStrictAboutTestsThatDoNotTestAnything="true"
+  beStrictAboutTodoAnnotatedTests="true"
   bootstrap="bootstrap.php"
   convertErrorsToExceptions="true"
   convertNoticesToExceptions="true"
   convertWarningsToExceptions="true"
-  strict="true"
+  enforceTimeLimit="true"
   >
   <testsuite name="Sabre_HTTP">
       <directory>HTTP/</directory>