Move OSS-Fuzz target file under tests

[mschwager]

Hi again!

I'm back working on fuzzing and trying to learn more about OSS-Fuzz. After my initial research into this project, I'm now hoping to promote it to OSS-Fuzz's "initial integration" tier. This requires: "Fuzz targets need to be checked into their upstream repository and integrated into the build system with sanitizer support."

I think this also helps with project developers who'd like to run the fuzz tests, but not necessarily install all the OSS-Fuzz infrastructure. loads_fuzzer.py can be run locally with the following commands...

First, install the fuzz dependencies:

python -m pip install -e .[fuzz]

Next, run the fuzz tests:

python tests/fuzzers/loads_fuzzer.py

This should produce output like the following:

INFO: Using preloaded libfuzzer
INFO: Running with entropic power schedule (0xFF, 100).
INFO: Seed: 2252594525
INFO: Loaded 1 modules   (4545 inline 8-bit counters): 4545 [0xffffb9d38c20, 0xffffb9d39de1), 
INFO: Loaded 1 PC tables (4545 PCs): 4545 [0xffffb9d39de8,0xffffb9d4b9f8), 
INFO: -max_len is not provided; libFuzzer will not generate inputs larger than 4096 bytes
INFO: A corpus is not provided, starting from an empty corpus
#2	INITED cov: 123 ft: 124 corp: 1/1b exec/s: 0 rss: 61Mb
	NEW_FUNC[1/1]: 0xffffb9cdb030 in decode_bytestring /app/cbor2/source/decoder.c:686
#5	NEW    cov: 132 ft: 141 corp: 2/2b lim: 4 exec/s: 0 rss: 62Mb L: 1/1 MS: 3 CrossOver-CopyPart-ChangeBit-
	NEW_FUNC[1/1]: 0xffffb9ce0ce8 in string_namespace_add /app/cbor2/source/decoder.c:512
...

Let me know if you'd like loads_fuzzer.py to live in a different location in the repository.

[coveralls]

coverage: 93.542%. remained the same
when pulling 3284769 on mschwager:mschwager-fuzz-target
into e1b65f2 on agronholm:master.

[coveralls]

coverage: 93.542%. remained the same
when pulling 88e4b90 on mschwager:mschwager-fuzz-target
into e1b65f2 on agronholm:master.