fix(test): use the same value for reldir v1+v2 (#11070)

## Problem

part of https://github.com/neondatabase/neon/issues/11067

My observation is that with the current value of settings, x86-v1
usually takes 30s, arm-v1 1m30s, x86-v2 1m, arm-v2 3m. But sometimes the
system could run too slow and cause test to timeout on arm with reldir
v2.

While I investigate what's going on and further improve the performance,
I'd like to set both of them to use the same test input, so that it
doesn't timeout and we don't abuse this test case as a performance test.

## Summary of changes

Use the same settings for both test cases.

Signed-off-by: Alex Chi Z <chi@neon.tech>

test_runner/regress/test_pg_regress.py

@@ -375,12 +375,8 @@ def test_tx_abort_with_many_relations(
 
     # How many relations: this number is tuned to be long enough to take tens of seconds
     # if the rollback code path is buggy, tripping the test's timeout.
-    if reldir_type == "v1":
-        n = 4000
-        step = 4000
-    else:
-        n = 20000
-        step = 5000
+    n = 5000
+    step = 2500
 
     def create():
         # Create many relations