build: disable tests by default

Having them autodetect is a bad idea since it would link cmocka in the main
mpv binary (which users don't want).

(cherry picked from commit ca0b33e9b1)

wscript

@@ -84,6 +84,7 @@ build_options = [
         'name': '--test',
         'desc': 'test suite (using cmocka)',
         'func': check_pkg_config('cmocka >= 0.4.1'),
+        'default': 'disable',
     }, {
         'name': '--clang-database',
         'desc': 'generate a clang compilation database',