g-api-py-oauthlib: refactor fuzzer (#8520)
g-api-py-oauthlib: refactor fuzzer (#8520)

In an effort to avoid spurious errors e.g.
- https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=50820
- https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=51094
google/oss-fuzz,google/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,google/oss-fuzz,skia-dev/oss-fuzz,google/oss-fuzz,google/oss-fuzz,google/oss-fuzz,skia-dev/oss-fuzz,google/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,google/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,google/oss-fuzz,google/oss-fuzz,google/oss-fuzz