pgsql: Improve test coverage of ginvacuum.c.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4c51a2d1e4b750bc11b8de9a85b079a14f798741

Modified Files
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src/test/regress/expected/gin.out | 21 +++++++++++++++++++++
src/test/regress/sql/gin.sql      |  8 ++++++++
2 files changed, 29 insertions(+)