fix flaky test EvictToNvmExpired and ReadFromNvmExpired

Summary:
The NvmCacheTest EvictToNvmExpired and ReadFromNvmExpired are relying on the TTL. However, the
reaper is enabled by default, the test could fail if it races with the reaper whose default interval
is the same as 5s https://fburl.com/testinfra/dyr5ovwn

Reviewed By: therealgymmy

Differential Revision: D49751136

fbshipit-source-id: bc388eabd68b144b40336744e4114d4e1bbdefb6

Author: Jaesoo Lee

cachelib/allocator/nvmcache/tests/NvmCacheTests.cpp

@@ -269,7 +269,10 @@ TEST_F(NvmCacheTest, EvictToNvmGetCheckCtime) {
 }
 
 TEST_F(NvmCacheTest, EvictToNvmExpired) {
-  auto& nvm = this->cache();
+  // Test with TTL, so the reaper should be disabled
+  auto& config = this->getConfig();
+  config.reaperInterval = std::chrono::milliseconds(0);
+  auto& nvm = this->makeCache();
   auto pid = this->poolId();
 
   const uint32_t ttl = 5; // 5 second ttl
@@ -294,7 +297,10 @@ TEST_F(NvmCacheTest, EvictToNvmExpired) {
 }
 
 TEST_F(NvmCacheTest, ReadFromNvmExpired) {
-  auto& nvm = this->cache();
+  // Test with TTL, so the reaper should be disabled
+  auto& config = this->getConfig();
+  config.reaperInterval = std::chrono::milliseconds(0);
+  auto& nvm = this->makeCache();
   auto pid = this->poolId();
 
   const uint32_t ttl = 5; // 5 second ttl