AsyncMessageSequence sees timeouts on oss-fuzz pretty often, but the
other just saw one and raised it to a bug, so put a limit on how many
messages the tests will read/decode to try and avoid the timeouts.
Fixes https://g-issues.oss-fuzz.com/issues/480506473
In Binary, TextFormat, and JSON, a leading byte is not value, so leverage that
to indicate that the next byte should be used as bits for decoding options.
```
FuzzTesting/Sources/FuzzAsyncMessageSequence/main.swift:27:21: warning: immutable value 'msg' was never used; consider replacing with '_' or removing it
for try await msg in decoding {
^~~
_
```
Thomas Van Lenten
Tim Kientzle