Fix test bugs, expand corpus

src/core/ext/transport/chttp2/transport/hpack_parser.c

src/core/lib/transport/metadata.h

test/core/end2end/fuzzers/api_fuzzer.c

...pi_fuzzer_corpus/07aa7d6c71878eb78b25ca12d79082f70ae7f64c

...pi_fuzzer_corpus/3465fb573ac3c59a0804aadeba2f205870abcc3d

...pi_fuzzer_corpus/595603f4ed37e3716cbe53b3ef180e5cdf8005f0

...pi_fuzzer_corpus/6186bfc21ff7df3982e5d9757e5c7160da0f493a

...pi_fuzzer_corpus/8a912877743b165b233303efaf502f5092b3c5b0

...pi_fuzzer_corpus/a8f87a7038125bd0e3b753c2a42ebdc3e4c75cba

...pi_fuzzer_corpus/bc6770a9bad24599ea4970735e9b17702a12b651

...zer_corpus/crash-7ca23a3e10cdbf579cf81a50e51af358f86631eb

...pi_fuzzer_corpus/d712d007679af5438c7bda723ddc724c2e57b0c1

tools/run_tests/tests.json