zlib: do not coalesce multiple `.flush()` calls

This is an approach to address the issue linked below. Previously,
when `.write()` and `.flush()` calls to a zlib stream were interleaved
synchronously (i.e. without waiting for these operations to finish),
multiple flush calls would have been coalesced into a single flushing
operation.

This patch changes behaviour so that each `.flush()` all corresponds
to one flushing operation on the underlying zlib resource, and the
order of operations is as if the `.flush()` call were a `.write()`
call.

One test had to be removed because it specifically tested the previous
behaviour.

As a drive-by fix, this also makes sure that all flush callbacks are
called. Previously, that was not the case.

Fixes: https://github.com/nodejs/node/issues/28478

PR-URL: https://github.com/nodejs/node/pull/28520
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

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