I do not know the test framework.
Libgit2 relies on the Clar test framework.
Does the messages at the end mean that only the diff::workdir::submodules failed?
Yes. More precisely, it points you to the failing assertion
Is it a known situation?
No. The libgit2 team works hard to always have the tests passing.
I've tried locally and I cannot reproduce this issue. This gist contains the build and test run against libgit2@4ae29053.
This has been run on Win 7 (32 bits) platform against Visual Studio 2010 as well.
The only difference I see is that you're running an older version of Python, but that shouldn't be a problem. Clar relies on Python to generate the test suite (more info about this in the README file of the project) and supports Python back to version 2.5.
If you can successfully reproduce this failure, please open an issue in the libgit2 issue tracker as this may require a more deeper troubleshooting that may not fit to the StackOverflow format.