Add DOS test that sends large transactions (#20624)

Problem
We need a test that stress-tests the network using large transactions, including sending multiple large transactions that reference independent sets of accounts, so they can be executed in parallel.

Summary of Changes
Adds such a test. Also adds a version of the tuner program that runs for a configurable number of iterations.

programs/bpf/c/src/tuner-variable-iterations/tuner-variable-iterations.c

@@ -0,0 +1,37 @@
+/**
+ * @brief Compute budget tuner program.  Spins in a loop for the specified number of iterations
+ * (or for UINT64_MAX iterations if 0 is specified for the number of iterations), in order to consume
+ * a configurable amount of the budget.
+ *
+ * Care should be taken because the compiler might optimize out the mechanism
+ * you are trying to tune.
+ */
+
+#include <solana_sdk.h>
+
+#define NUM_KA 1
+
+extern uint64_t entrypoint(const uint8_t *input) {
+  SolAccountInfo ka[NUM_KA];
+  SolParameters params = (SolParameters){.ka = ka};
+  if (!sol_deserialize(input, &params, SOL_ARRAY_SIZE(ka))) {
+    return ERROR_INVALID_ARGUMENT;
+  }
+
+  size_t current = 1;
+
+  uint64_t iterations = params.data[0] * 18;
+  iterations = iterations == 0 ? UINT64_MAX : iterations;
+  size_t rand = params.data[1];
+  size_t account_index = rand % params.ka_num;
+  uint8_t *val = (uint8_t *)ka[account_index].data;
+  uint64_t memory_size = ka[account_index].data_len;
+
+  for (uint64_t i = 0; i < iterations; i++) {
+    {
+      *val ^= val[current % memory_size];
+      current = current * 76510171 + 47123;
+    }
+  }
+  return *val;
+}