pgsql: Reduce length of GIN predicate locking isolation test suite

Reduce length of GIN predicate locking isolation test suite

Isolation test suite of GIN predicate locking was criticized for being too slow,
especially under Valgrind. This commit is intended to accelerate it. Tests are
simplified in the following ways.

1) Amount of data is reduced. We're now close to the minimal amount of data,
which produces at least one posting tree and at least two pages of entry
tree.
2) Three isolation tests are merged into one.
3) Only one tuple is queried from posting tree. So, locking of index is the
same, but tuple locks are not propagated to relation lock. Also, it is
faster.
4) Test cases itself are simplified. Now each test case run just one INSERT
and one SELECT involving GIN, which either conflict or not.

Discussion: https://postgr.es/m/20181204000740.ok2q53nvkftwu43a%40alap3.anarazel.de
Reported-by: Andres Freund
Tested-by: Andrew Dunstan
Author: Alexander Korotkov
Backpatch-through: 11

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fcdda202bcf8fa71e1b919f62d3bb96f610ff25f

Modified Files
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.../expected/predicate-gin-fastupdate.out | 30 -
.../isolation/expected/predicate-gin-nomatch.out | 15 -
src/test/isolation/expected/predicate-gin.out | 783 +++++++--------------
src/test/isolation/isolation_schedule | 2 -
.../isolation/specs/predicate-gin-fastupdate.spec | 49 --
.../isolation/specs/predicate-gin-nomatch.spec | 35 -
src/test/isolation/specs/predicate-gin.spec | 174 ++---
7 files changed, 320 insertions(+), 768 deletions(-)