tests: default interpreted proto in tests (#10079)

## Problem

We aren't using the sharded interpreted wal receiver protocol in all
tests.

## Summary of changes

Default to the interpreted protocol.

test_runner/fixtures/neon_fixtures.py

@@ -435,7 +435,10 @@ def __init__(
 
         self.pageserver_virtual_file_io_mode = pageserver_virtual_file_io_mode
 
-        self.pageserver_wal_receiver_protocol = pageserver_wal_receiver_protocol
+        if pageserver_wal_receiver_protocol is not None:
+            self.pageserver_wal_receiver_protocol = pageserver_wal_receiver_protocol
+        else:
+            self.pageserver_wal_receiver_protocol = PageserverWalReceiverProtocol.INTERPRETED
 
         assert test_name.startswith(
             "test_"