pgsql: Create waitfuncs.c for pg_isolation_test_session_is_blocked().

Create waitfuncs.c for pg_isolation_test_session_is_blocked().

The next commit makes the function inspect an additional non-lock
contention source, so it no longer fits in lockfuncs.c.

Reviewed by Robert Haas.

Discussion: https://postgr.es/m/20240512232923.aa.nmisch@google.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/abfbd13af0e971e8789bc89e7c83ad53a85fa74b

Modified Files
--------------
src/backend/utils/adt/Makefile | 1 +
src/backend/utils/adt/lockfuncs.c | 79 --------------------------------
src/backend/utils/adt/meson.build | 1 +
src/backend/utils/adt/waitfuncs.c | 96 +++++++++++++++++++++++++++++++++++++++
4 files changed, 98 insertions(+), 79 deletions(-)