Makefile: Build libgit2 as part of prepare-tests

Given that libgit2 is required to run our Go test suite, it is being built as part of our test-go Makefile target. Given that we have a prepare-tests target, this is a bit awkward as a developer may righlty assume that after having run prepare-tests, that he is able to now run all of our tests. But the dev will be quick to note that all targets which depend on libgit2 will fail now.

Let's improve this by moving the libgit2 dependency from test-go to prepare-tests.