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/README.md

@@ -15,12 +15,22 @@ Prerequisites:
 
 ### Test Organization
 
-The tests are divided into a few batches, such that each batch takes roughly
-the same amount of time. The batches can be run in parallel, to minimize total
-runtime. Currently, there are only two batches:
+Regression tests are in the 'regress' directory. They can be run in
+parallel to minimize total runtime. Most regression test sets up their
+environment with its own pageservers and safekeepers (but see
+`TEST_SHARED_FIXTURES`).
 
-- test_batch_pg_regress: Runs PostgreSQL regression tests
-- test_others: All other tests
+'pg_clients' contains tests for connecting with various client
+libraries. Each client test uses a Dockerfile that pulls an image that
+contains the client, and connects to PostgreSQL with it. The client
+tests can be run against an existing PostgreSQL or Neon installation.
+
+'performance' contains performance regression tests. Each test
+exercises a particular scenario or workload, and outputs
+measurements. They should be run serially, to avoid the tests
+interfering with the performance of each other. Some performance tests
+set up their own Neon environment, while others can be run against an
+existing PostgreSQL or Neon environment.
 
 ### Running the tests