Update test case

Author: tdrwenski

tests/performance/test_cases.json

@@ -2,9 +2,9 @@
     {
         "simulation_name": "nlevels_8_8_8_8",
         "number_of_processes": [
-            16
+            4
         ],
-        "repetitions": 20
+        "repetitions": 100
     },
     {
         "simulation_name": "nlevels_32_32_32_32",