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This test fails on Windows because it tries to change the working folder to an empty string.
Anyway, even changing:
 os.chdir(basepath)
with:
 os.chdir(basepath or '.')
it fails because the subprocess tries to execute an ELF file ('Modules/_testembed') which Windows isn't able to handle.

I don't really understand why you need this. Under normal setups, the test is already properly skipped under Windows, since there's no rule to build the "Modules/_testembed" with MSVC.
Or are you talking about another kind of setup? Cygwin? mingw?

Unfortunately this test isn't skip on normal setup (using VisualStudio 2008 Express). I've updated the clone just a few minutes ago, rebuilt Python (in Debug mode), and that's what happened:
D:\CPython>PCbuild\python_d.exe Lib\test\test_capi.py
test_instancemethod (__main__.CAPITest) ... ok
test_memoryview_from_NULL_pointer (__main__.CAPITest) ... ok
test_no_FatalError_infinite_loop (__main__.CAPITest) ... ok
test_pendingcalls_non_threaded (__main__.TestPendingCalls) ... ok
test_pendingcalls_threaded (__main__.TestPendingCalls) ... ok
test (__main__.Test6012) ... ok
test_subinterps (__main__.EmbeddingTest) ... ERROR
======================================================================
ERROR: test_subinterps (__main__.EmbeddingTest)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "Lib\test\test_capi.py", line 155, in test_subinterps
 os.chdir(basepath)
WindowsError: [Error 123] La sintassi del nome del file, della directory o del volume non è corretta: ''
----------------------------------------------------------------------
Ran 7 tests in 4.587s
FAILED (errors=1)
Traceback (most recent call last):
 File "Lib\test\test_capi.py", line 212, in <module>
 test_main()
 File "Lib\test\test_capi.py", line 176, in test_main
 support.run_unittest(CAPITest, TestPendingCalls, Test6012, EmbeddingTest)
 File "D:\CPython\lib\test\support.py", line 1280, in run_unittest
 _run_suite(suite)
 File "D:\CPython\lib\test\support.py", line 1263, in _run_suite
 raise TestFailed(err)
test.support.TestFailed: Traceback (most recent call last):
 File "Lib\test\test_capi.py", line 155, in test_subinterps
 os.chdir(basepath)
WindowsError: [Error 123] La sintassi del nome del file, della directory o del volume non è corretta: ''
[90425 refs]
The patch ensures that this test isn't always executed on Windows, as expected.

New changeset 7c60c1b41da9 by Antoine Pitrou in branch '3.2':
Issue #12407: Explicitly skip test_capi.EmbeddingTest under Windows.
http://hg.python.org/cpython/rev/7c60c1b41da9 

New changeset 5cae52417b9d by Antoine Pitrou in branch 'default':
Issue #12407: Explicitly skip test_capi.EmbeddingTest under Windows.
http://hg.python.org/cpython/rev/5cae52417b9d 

Fixed, thank you.