pgsql: Try again to add test coverage for pg_combinebackup w/tablespace

Try again to add test coverage for pg_combinebackup w/tablespaces.

My previous attempt to add this had to be reverted in commit
82023d47de9e262730b1f9b4ea77fae201a89d0a. I've revised the problematic
code a bit; hopefully it is OK now.

Discussion: http://postgr.es/m/CA+Tgmobiv1QJR5PEJoDKeZDrJHZFRmi4XmWOqufN49DJj-3e2g@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/89ad3e1316bf0374f9e57456bf2888d14bf070f6

Modified Files
--------------
src/bin/pg_basebackup/t/010_pg_basebackup.pl      | 21 +----
src/bin/pg_combinebackup/t/002_compare_backups.pl | 37 ++++++++-
src/test/perl/PostgreSQL/Test/Cluster.pm          | 96 +++++++++++++++++++++--
3 files changed, 126 insertions(+), 28 deletions(-)