Test streams should use fail_count

[vinniefalco]

One of the main test modes of test::stream is that it can fail on the Nth operation, for increasing values of N. This ensures that every possible branch in the code being tested is exercised:

https://github.com/boostorg/beast/blob/97ece405b8127e1d4767a8f63b82478d5637b9ec/include/boost/beast/_experimental/test/fail_count.hpp#L31

The tests should be constructing the test::stream with a fail_count, and then running the same test over and over again in a loop with incrementing fail_count until the test passes.

[mzimbres]

PR #151 has removed the resp3::read and async_read functions and therefore we don't need the beast test stream anymore. The way the parser works now enables testing without any concept of stream.