pgsql: Make more stable regression tests of dummy_index_am for string v

Make more stable regression tests of dummy_index_am for string validations

Several buildfarm members (crake, loach and spurfowl) are complaining
about two queries looking up at pg_class.reloptions which trigger the
validation routines for string reloptions with default values. This
commit limits the routines to be triggered only when building an index
with all custom options set in CREATE INDEX, which is sufficient for the
coverage.

Introduced by 640c198.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e0afac124ec7026a49909436ebcfc2bd999852a8

Modified Files
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src/test/modules/dummy_index_am/expected/reloptions.out | 8 ++++++--
src/test/modules/dummy_index_am/sql/reloptions.sql | 10 +++++++---
2 files changed, 13 insertions(+), 5 deletions(-)