pg_stat_statements: more test coverage

pg_stat_statements has some significant gaps in test coverage, 
especially around the serialization of data around server restarts, so I 
wrote a test for that and also made some other smaller tweaks to 
increase the coverage a bit.  These patches are all independent of each 
other.

After that, the only major function that isn't tested is gc_qtexts(). 
Maybe a future project.