Replace tests of ALTER DATABASE RESET TABLESPACE.

Replace tests of ALTER DATABASE RESET TABLESPACE.

This unblocks rejection of that syntax.  One copy was a misspelling of
"SET TABLESPACE pg_default" that instead made no persistent changes.
The other copy just needed to populate a DATABASEOID syscache entry.
This slightly raises database.sql test coverage of catcache.c, while
dbcommands.c coverage remains the same.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1802710.1757608564@sss.pgh.pa.us