Add resource consume test funcs (#6747)

## Problem

Building on #5875 to add handy test functions for autoscaling.

Resolves #5609

## Summary of changes

This PR makes the following changes to #5875:
- Enable `neon_test_utils` extension in the compute node docker image,
so we could use it in the e2e tests (as discussed with @kelvich).
- Removed test functions related to disk as we don't use them for
autoscaling.
- Fix the warning with printf-ing unsigned long variables.

---------

Co-authored-by: Heikki Linnakangas <heikki@neon.tech>

test_runner/sql_regress/expected/neon-test-utils.out

@@ -0,0 +1,28 @@
+-- Test the test utils in pgxn/neon_test_utils. We don't test that
+-- these actually consume resources like they should - that would be
+-- tricky - but at least we check that they don't crash.
+CREATE EXTENSION neon_test_utils;
+select test_consume_cpu(1);
+ test_consume_cpu 
+------------------
+ 
+(1 row)
+
+select test_consume_memory(20); -- Allocate 20 MB
+ test_consume_memory 
+---------------------
+ 
+(1 row)
+
+select test_release_memory(5);  -- Release 5 MB
+ test_release_memory 
+---------------------
+ 
+(1 row)
+
+select test_release_memory();   -- Release the remaining 15 MB
+ test_release_memory 
+---------------------
+ 
+(1 row)
+