parallel bitmapscan isn't exercised in regression tests

Hi,

The parallel code-path isn't actually exercised in the tests added in
[1], as evidenced by [2] (they just explain).  That imo needs to be
fixed.

Greetings,

Andres Freund

[1] https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=f35742ccb7aa53ee3ed8416bbb378b0c3eeb6bb9
[2] https://coverage.postgresql.org/src/backend/executor/nodeBitmapHeapscan.c.gcov.html