Speedup the slow test

Previously took >60 seconds to run and over 97% of that time was in "constraint_generation/src/execute.rs::execute_template_call" which is part of the original Circom compiler implementation (so outside of our scope). This version runs much faster but still exhibits the same crashing behavior as it did prior to commit `a1ce494`.

circom/tests/calls/van-423.circom

@@ -480,10 +480,10 @@ function mod_exp(n, k, a, p, e) {
 template Test() {
 	var out[50];
 	var n = 55;
-	var k = 7;
-	var a[k] = [1, 2, 3, 4, 5, 6, 7];
-	var p[k] = [35747322042231467, 36025922209447795, 1084959616957103, 7925923977987733, 16551456537884751, 23443114579904617, 1829881462546425];
-	var e[k] = [7, 8, 9, 10, 11, 12, 13];
+	var k = 1;
+	var a[k] = [3];
+	var p[k] = [35747322042231467];
+	var e[k] = [7];
 
 	out = mod_exp(n, k, a, p, e);
 }