pgsql: Add regression test listing all the possible tuples in pg_stat_i

Add regression test listing all the possible tuples in pg_stat_io

pg_stat_io returns a set of tuples based on a combination of three
properties (BackendType, IOObject and IOContext) and
pgstat_tracks_io_object() to decide if a BackendType should return a
tuple based on a pair made of an IOObject and an IOContext.

This commit adds a regression test to track all the combinations
supported. This is useful to know which tuples are relevant when adding
a new BackendType to the set or when touching pgstat_tracks_io_object(),
and I have noticed while playing with this area that it is not
complicated to break it without the regression tests noticing a
difference in some cases.

Reviewed-by: Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com>
Discussion: https://postgr.es/m/Z8exfAehbVbEKXW5@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8b532771a09975f274b35823aa355d8ca90c77db

Modified Files
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src/test/regress/expected/stats.out | 78 +++++++++++++++++++++++++++++++++++++
src/test/regress/sql/stats.sql | 6 +++
2 files changed, 84 insertions(+)