Fix missing subst_target_sil_opt_mock_sdk in lit.cfg

Author: MaxDesiatov

test/lit.cfg

@@ -1382,6 +1382,8 @@ elif run_os == 'wasi':
     config.target_sil_opt = (
         '%s -target %s %s %s %s' %
         (config.sil_opt, config.variant_triple, resource_dir_opt, mcp_opt, config.sil_test_options))
+    subst_target_sil_opt_mock_sdk = config.target_sil_opt
+    subst_target_sil_opt_mock_sdk_after = ""
     config.target_swift_symbolgraph_extract = ' '.join([
         config.swift_symbolgraph_extract,
         '-target', config.variant_triple,