pgsql: Fix default_tablespace usage for partitioned tables

Fix default_tablespace usage for partitioned tables

In commit 87259588d0ab I (Álvaro) tried to rationalize the determination
of tablespace to use for partitioned tables, but failed to handle the
default_tablespace case.  Repair and add proper tests.

Author: Amit Langote, Rushabh Lathia
Reported-by: Rushabh Lathia
Reviewed-by: Amit Langote, Álvaro Herrera
Discussion: https://postgr.es/m/CAGPqQf0cYjm1=rjxk_6gU0SjUS70=yFUAdCJLwWzh9bhNJnyVg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a36c84c3e4a9bee6baa740848f67a5db3fa279b0

Modified Files
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src/backend/commands/tablecmds.c          |  8 +++--
src/test/regress/input/tablespace.source  | 34 +++++++++++++++++----
src/test/regress/output/tablespace.source | 50 ++++++++++++++++++++++++-------
3 files changed, 73 insertions(+), 19 deletions(-)