tests: Enable python based tests by default

They have been around for quite some time now but it's easy to
accidentally not run them because they get disabled when the
dependencies are not all satisfied.

This enables pytests by default which forces one to explicitly disable
them if they are unwanted.

meson_options.txt

@@ -40,7 +40,7 @@ option('installed-tests',
         description: 'Enable installation of some test cases')
 option('pytest',
         type: 'feature',
-        value: 'auto',
+        value: 'enabled',
         description: 'Enable the pytest-based test suite')
 option('man-pages',
        type: 'feature',