libgit2_clar is now libgit2_tests
in #6083 the test runner was renamed to libgit2_tests,
but not all references to the old name were updated.
this change changes all of them to use the new name.
fuzzers: add object parsing fuzzer
Add a simple fuzzer that exercises our object parser code. The fuzzer
is quite trivial in that it simply passes the input data directly to
`git_object__from_raw` for each of the four object types.