libxml2: fix test method (#37242)

This was discovered using #34236

var/spack/repos/builtin/packages/libxml2/package.py

@@ -105,7 +105,7 @@ def test(self):
         """Perform smoke tests on the installed package"""
         # Start with what we already have post-install
         tty.msg("test: Performing simple import test")
-        self.import_module_test()
+        self.builder.import_module_test()
 
         data_dir = self.test_suite.current_test_data_dir