Columnar Memory Test Fails on Marlin

[am_memory](https://github.com/citusdata/citus/blob/release-10.0/src/test/regress/sql/am_memory.sql) test fails on Marlin.

The failing part is this select query at the end of the test:
```
SELECT
(SELECT max(memusage) < 8 * 1024 * 1024 FROM t WHERE tag='large batch') AS large_batch_ok,
(SELECT max(memusage) < 8 * 1024 * 1024 FROM t WHERE tag='first batch') AS first_batch_ok;
```

After removing `< 8 * 1024 * 1024` and running on the same cluster again the output was:

```
┌────────────────┬────────────────┐
│ large_batch_ok │ first_batch_ok │
├────────────────┼────────────────┤
│ 13882232 │ 9140088 │
└────────────────┴────────────────┘
(1 row)
```

This output changed between different runs but all would fail the `< 8 * 1024 * 1024` check.

Current name of the test is columnar_memory.sql.