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.

Nobody knows what CLAR is.  The test building option should be
`BUILD_TESTS`.

This change is the first in a series to add support for git's
commit-graph. This should speed up commit graph traversals by avoiding
object parsing and allowing some operations to terminate earlier.

Part of: #5757

This change is the first in a series to add support for git's
multi-pack-index. This should speed up large repositories significantly.

Part of: #5399

I was looking at this code anyway because the sr.ht people nerdsniped
me, and it gave me that "I should fuzz this" feeling. So have a 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.

This is a direct copy of the code from google/oss-fuzz, written by
Nelson Elhage (@nelhage). Note that due to the ".cc" ending, the file
will not yet be picked up by the build system. This is intended, as
currently that file is partly written in C++, requiring a conversion to
C.

Our layout uses names like "examples" or "tests" which is why the "fuzz"
directory doesn't really fit in here. Rename the directory to be called
"fuzzers" instead. Furthermore, we rename the fuzzer "fuzz_packfile_raw"
to "packfile_raw_fuzzer", which is also in line with the already
existing fuzzer at google/oss-fuzz.

While at it, rename the "packfile_raw" fuzzer to instead just be called
"packfile" fuzzer.