[libc++] Try fixing flaky fs.op.space test on FreeBSD (#213988)

As part of this test, we check the capacity available on the filesystem
and compare it against an expected capacity. However, on some
filesystems, the capacity is actually computed and may change depending
on filesystem usage. This creates a race condition since the disk may be
filling up (by e.g. other running tests) between the two calls.

For that same reason, other checks for the available size were using an
approximate equality up to a delta. Use the same approach for the
capacity here.

GitOrigin-RevId: 4a19c1129437bde0f37fc58f975f863e01726a7a