fix(c-api) Don't drain the entire captured stream at first read

[Hywan]

Description

We use VecDeque::drain to read the captured stream, zipped with the
given buffer. We could expect that only the yielded items from the
drain will be removed, but actually no. Reading the
documentation:

Note 1: The element range is removed even if the iterator is not
consumed until the end.

So by using a range like .. (as we do) will drain the entire captured stream,
whatever we read from it. Said differently, if the given buffer length
is smaller than the captured stream, the first read will drain the
entire captured stream.

This patch fixes the problem by specifying a better range:
..min(inner_buffer.len(), oc.buffer.len()).

With this new range, it's actually useless to increment
num_bytes_written, we already know ahead of time the amount of bytes
we are going to read. Consequently, the patch simplifies this code a
little bit more.

This patch also updates the test-wasi integration test. Previously,
stdout wasn't captured, so the loop using wasi_env_read_stdout was
doing nothing. Now we have the following output:

Initializing...
Loading binary...
Compiling module...
Setting up WASI...
Instantiating module...
Extracting export...
found 2 exports
Calling export...
Evaluating "function greet(name) { return JSON.stringify('Hello, ' + name); }; print(greet('World'));"
WASI Stdout: `"Hello, World"
`
Shutting down...
Done.

Review

• Add a short description of the the change to the CHANGELOG.md file

[Hywan]

bors r+

[bors]

Build succeeded:

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