tests: add optional cursor to log_contains + fix truthiness issues in callers (#6960)

Extracted from https://github.com/neondatabase/neon/pull/6953

Part of https://github.com/neondatabase/neon/issues/5899

Core Change
-----------

In #6953, we need the ability to scan the log _after_ a specific line
and ignore anything before that line.

This PR changes `log_contains` to returns a tuple of `(matching line,
cursor)`.
Hand that cursor to a subsequent `log_contains` call to search the log
for the next occurrence of the pattern.

Other Changes
-------------

- Inspect all the callsites of `log_contains` to handle the new tuple
return type.
- Above inspection unveiled many callers aren't using `assert
log_contains(...) is not None` but some weaker version of the code that
breaks if `log_contains` ever returns a not-None but falsy value. Fix
that.
- Above changes unveiled that `test_remote_storage_upload_queue_retries`
was using `wait_until` incorrectly; after fixing the usage, I had to
raise the `wait_until` timeout. So, maybe this will fix its flakiness.

test_runner/regress/test_tenant_relocation.py

@@ -495,7 +495,7 @@ def test_emergency_relocate_with_branches_slow_replay(
         assert cur.fetchall() == [("before pause",), ("after pause",)]
 
     # Sanity check that the failpoint was reached
-    assert env.pageserver.log_contains('failpoint "wal-ingest-logical-message-sleep": sleep done')
+    env.pageserver.assert_log_contains('failpoint "wal-ingest-logical-message-sleep": sleep done')
     assert time.time() - before_attach_time > 5
 
     # Clean up
@@ -632,7 +632,7 @@ def test_emergency_relocate_with_branches_createdb(
         assert query_scalar(cur, "SELECT count(*) FROM test_migrate_one") == 200
 
     # Sanity check that the failpoint was reached
-    assert env.pageserver.log_contains('failpoint "wal-ingest-logical-message-sleep": sleep done')
+    env.pageserver.assert_log_contains('failpoint "wal-ingest-logical-message-sleep": sleep done')
     assert time.time() - before_attach_time > 5
 
     # Clean up