Adjust expected VNG size in export test

[philrz]

While #3062 addressed one of our recent failures in Zui CI due to a Zed change, this recent Actions run shows that the very next Zed change at commit ce9626e was ready to cause a CI failure next. Assuming it's no surprise that the changes in brimdata/zed#5118 would cause a change in the VNG file size, this PR adjusts the test to expect the new size.

$ zq -version
Version: v1.15.0-16-g183e432f

$ zq -f vng 'sort ts, _path' ./apps/zui/src/test/shared/data/sample.zeektsv | wc -c
    7983

$ zq -version
Version: v1.15.0-17-gce9626eb

$ zq -f vng 'sort ts, _path' ./apps/zui/src/test/shared/data/sample.zeektsv | wc -c
    7972