Reorganize python tests.

Merge batch_others and batch_pg_regress. The original idea was to
split all the python tests into multiple "batches" and run each batch
in parallel as a separate CI job. However, the batch_pg_regress batch
was pretty short compared to all the tests in batch_others. We could
split batch_others into multiple batches, but it actually seems better
to just treat them as one big pool of tests and use pytest's handle
the parallelism on its own. If we need to split them across multiple
nodes in the future, we could use pytest-shard or something else,
instead of managing the batches ourselves.

Merge test_neon_regress.py, test_pg_regress.py and test_isolation.py
into one file, test_pg_regress.py. Seems more clear to group all
pg_regress-based tests into one file, now that they would all be in
the same directory.

test_runner/sql_regress/parallel_schedule

@@ -0,0 +1,10 @@
+# ----------
+# Like in PostgreSQL src/test/regress/parallel_schedule, we put no
+# more than twenty tests in any one parallel group; this limits the
+# number of connections needed to run the tests.
+# ----------
+
+test: neon-cid
+test: neon-rel-truncate
+test: neon-clog
+test: neon-vacuum-full