t5351: avoid relying on `core.fsyncMethod = batch` to be supported

t5351: avoid relying on `core.fsyncMethod = batch` to be supported

On FreeBSD, this mode is not supported. But since 3a251bac0d1a (trace2:
only include "fsync" events if we git_fsync(), 2022-07-18) t5351 will
fail if this mode is unsupported.

Let's address this in the minimal fashion, by detecting that that mode
is unsupported and expecting a different count of hardware flushes in
that case.

This fixes the CI/PR builds on FreeBSD again.

Note: A better way would be to test only what is relevant in t5351.6
"unpack big object in stream (core.fsyncmethod=batch)" again instead of
blindly comparing the output against some exact text. But that would
pretty much revert the idea of above-mentioned commit, and that commit
was _just_ accepted into Git's main branch so one must assume that it
was intentional.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>