Temporarily(?) remove BRIN isolation test.

Commit 2834855c added a not-very-carefully-thought-out isolation test
to check a BRIN index bug fix.  The test depended on the availability
of the pageinspect contrib module, which meant it did not work in
several common testing scenarios such as "make check-world".  It's not
clear whether we want a core test depending on a contrib module like
that, but in any case, failing to deal with the possibility that the
module isn't present in the installation-under-test is not acceptable.

Remove that test pending some better solution.

src/test/isolation/expected/brin-1.out

-Parsed test spec with 2 sessions
-
-starting permutation: s2check s1b s2b s1i s2summ s1c s2c s2check
-step s2check: SELECT * FROM brin_page_items(get_raw_page('brinidx', 2), 'brinidx'::regclass);
-itemoffset     blknum         attnum         allnulls       hasnulls       placeholder    value          
-
-1              0              1              f              f              f              {1 .. 1}       
-step s1b: BEGIN ISOLATION LEVEL REPEATABLE READ;
-step s2b: BEGIN ISOLATION LEVEL REPEATABLE READ; SELECT 1;
-?column?       
-
-1              
-step s1i: INSERT INTO brin_iso VALUES (1000);
-step s2summ: SELECT brin_summarize_new_values('brinidx'::regclass);
-brin_summarize_new_values
-
-1              
-step s1c: COMMIT;
-step s2c: COMMIT;
-step s2check: SELECT * FROM brin_page_items(get_raw_page('brinidx', 2), 'brinidx'::regclass);
-itemoffset     blknum         attnum         allnulls       hasnulls       placeholder    value          
-
-1              0              1              f              f              f              {1 .. 1}       
-2              1              1              f              f              f              {1 .. 1000}    
-
-starting permutation: s2check s1b s1i s2vacuum s1c s2check
-step s2check: SELECT * FROM brin_page_items(get_raw_page('brinidx', 2), 'brinidx'::regclass);
-itemoffset     blknum         attnum         allnulls       hasnulls       placeholder    value          
-
-1              0              1              f              f              f              {1 .. 1}       
-step s1b: BEGIN ISOLATION LEVEL REPEATABLE READ;
-step s1i: INSERT INTO brin_iso VALUES (1000);
-step s2vacuum: VACUUM brin_iso;
-step s1c: COMMIT;
-step s2check: SELECT * FROM brin_page_items(get_raw_page('brinidx', 2), 'brinidx'::regclass);
-itemoffset     blknum         attnum         allnulls       hasnulls       placeholder    value          
-
-1              0              1              f              f              f              {1 .. 1}       
-2              1              1              f              f              f              {1 .. 1000}    

src/test/isolation/isolation_schedule

@@ -36,7 +36,6 @@ test: skip-locked
 test: skip-locked-2
 test: skip-locked-3
 test: skip-locked-4
-test: brin-1
 test: drop-index-concurrently-1
 test: alter-table-1
 test: alter-table-2

src/test/isolation/specs/brin-1.spec

-# This test verifies that values inserted in transactions still in progress
-# are considered during concurrent range summarization (either using the
-# brin_summarize_new_values function or regular VACUUM).
-
-setup
-{
-    CREATE TABLE brin_iso (
-        value int
-    ) WITH (fillfactor=10);
-    CREATE INDEX brinidx ON brin_iso USING brin (value) WITH (pages_per_range=1);
-    -- this fills the first page
-    DO $$
-    DECLARE curtid tid;
-    BEGIN
-      LOOP
-        INSERT INTO brin_iso VALUES (1) RETURNING ctid INTO curtid;
-        EXIT WHEN curtid > tid '(1, 0)';
-      END LOOP;
-    END;
-    $$;
-    CREATE EXTENSION IF NOT EXISTS pageinspect;
-}
-
-teardown
-{
-    DROP TABLE brin_iso;
-}
-
-session "s1"
-step "s1b"		{ BEGIN ISOLATION LEVEL REPEATABLE READ; }
-step "s1i"		{ INSERT INTO brin_iso VALUES (1000); }
-step "s1c"		{ COMMIT; }
-
-session "s2"
-step "s2b"		{ BEGIN ISOLATION LEVEL REPEATABLE READ; SELECT 1; }
-step "s2summ"	{ SELECT brin_summarize_new_values('brinidx'::regclass); }
-step "s2c"		{ COMMIT; }
-
-step "s2vacuum"	{ VACUUM brin_iso; }
-
-step "s2check"	{ SELECT * FROM brin_page_items(get_raw_page('brinidx', 2), 'brinidx'::regclass); }
-
-permutation "s2check" "s1b" "s2b" "s1i" "s2summ" "s1c" "s2c" "s2check"
-permutation "s2check" "s1b" "s1i" "s2vacuum" "s1c" "s2check"