test: make target_codesign work on Windows

Assume that GNUWin32 tools are present and that we can just use echo to
stub out the command like on Linux.

Author: compnerd

test/lit.cfg

@@ -817,7 +817,7 @@ elif run_os in ['windows-msvc']:
                                                       config.swift_test_options, \
                                                       config.swift_frontend_test_options))
 
-    config.target_codesign = ''
+    config.target_codesign = 'echo'
 
     subst_target_swift_frontend_mock_sdk = config.target_swift_frontend
     subst_target_swift_frontend_mock_sdk_after = ''