ESQL: Use less data in test (elastic#128337)

This test is expected to complete a deeply abusive lookup join without
circuit breaking. It was sometimes circuit breaking. This lowers the
data we feed to the test so its slightly less abusive. It's still plenty
abusive.

Closes elastic#127365

Author: nik9000

test/external-modules/esql-heap-attack/src/javaRestTest/java/org/elasticsearch/xpack/esql/heap_attack/HeapAttackIT.java

@@ -653,7 +653,7 @@ public void testLookupExplosionManyMatches() throws IOException {
     }
 
     public void testLookupExplosionNoFetch() throws IOException {
-        int sensorDataCount = 7000;
+        int sensorDataCount = 6000;
         int lookupEntries = 10000;
         Map<?, ?> map = lookupExplosionNoFetch(sensorDataCount, lookupEntries);
         assertMap(map, matchesMap().extraOk().entry("values", List.of(List.of(sensorDataCount * lookupEntries))));