pgsql: Fix contrib/pageinspect's test for sequences.

Fix contrib/pageinspect's test for sequences.

I managed to break this test in two different ways in commit
05036a3155.

First, the output of the new call to tuple_data_split() on the test
sequence is dependent on endianness. This is fixed by setting a
special start value for the test sequence that produces the same
output regardless of the endianness of the machine.

Second, on versions older than v15, the new test case fails under
"force_parallel_mode = regress" with the following error:

ERROR: cannot access temporary tables during a parallel operation

This is because pageinspect's disk-accessing functions are
incorrectly marked PARALLEL SAFE on versions older than v15 (see
commit aeaaf520f4 for details). This one is fixed by changing the
test sequence to be permanent. The only reason it was previously
marked temporary was to avoid needing a DROP SEQUENCE command at
the end of the test. Unlike some other tests in this file, the use
of a permanent sequence here shouldn't result in any test
instability like what was fixed by commit e2933a6e11.

Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/ZuOKOut5hhDlf_bP%40nathan
Backpatch-through: 12

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0938a4ecda33ce0630dd4cb01466f8ef4c41f8b4

Modified Files
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contrib/pageinspect/expected/page.out | 5 +++--
contrib/pageinspect/sql/page.sql | 3 ++-
2 files changed, 5 insertions(+), 3 deletions(-)