pgsql: Improve readability of some tests in strings.sql

Improve readability of some tests in strings.sql

c251336 has added some tests to check if a toast relation should be
empty or not, hardcoding the toast relation name when calling
pg_relation_size().  pg_class.reltoastrelid offers the same information,
so simplify the tests to use that.

Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/20190403065949.GH3298@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/92c76021aed47daa61afd5c7d6f8201382dee203

Modified Files
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src/test/regress/expected/strings.out | 14 ++++++++------
src/test/regress/sql/strings.sql      |  6 ++++--
2 files changed, 12 insertions(+), 8 deletions(-)