Reduce ram_cache regression test scope for CI stability (#12737)

The ram_cache test was failing in CI when run with -R 3 due to the 256MB
cache size test running over 4 million iterations. This caused timeouts
or OOM kills in slower CI environments. Reduced the maximum cache size
from 256MB to 16MB and lowered the minimum regression level from
EXTENDED to NIGHTLY.

Fixes: #12728

Author: bneradt

src/iocore/cache/CacheTest.cc

@@ -658,8 +658,7 @@ test_RamCache(RegressionTest *t, RamCache *cache, const char *name, int64_t cach
 
 REGRESSION_TEST(ram_cache)(RegressionTest *t, int level, int *pstatus)
 {
-  // Run with -R 3 for now to trigger this check, until we figure out the CI
-  if (REGRESSION_TEST_EXTENDED > level) {
+  if (REGRESSION_TEST_NIGHTLY > level) {
     *pstatus = REGRESSION_TEST_PASSED;
     return;
   }
@@ -669,7 +668,12 @@ REGRESSION_TEST(ram_cache)(RegressionTest *t, int level, int *pstatus)
     *pstatus = REGRESSION_TEST_FAILED;
     return;
   }
-  for (int s = 20; s <= 28; s += 4) {
+
+  // Test cache sizes from 1MB to 16MB. The sample_size is cache_size >> 6, so
+  // the 16MB test runs 262K iterations which completes in reasonable time. This
+  // used to run with 256MB (s=28) at 4M+ iterations, which is too much for CI
+  // resulting in failures, either due to timeout or OOM issues.
+  for (int s = 20; s <= 24; s += 4) {
     int64_t cache_size = 1LL << s;
     *pstatus           = REGRESSION_TEST_PASSED;
     if (!test_RamCache(t, new_RamCacheLRU(), "LRU", cache_size) || !test_RamCache(t, new_RamCacheCLFUS(), "CLFUS", cache_size)) {