Improve test coverage of ginvacuum.c.
Add a test case that exercises vacuum's deletion of empty GIN
posting pages. Since this is a temp table, it should now work
reliably to delete a bunch of rows and immediately VACUUM.
Before the preceding commit, this would not have had the desired
effect, at least not in parallel regression tests.
Discussion: https://postgr.es/m/
3490536.
1598629609@sss.pgh.pa.us