Increase CTest timeout to 300s for CI stability

Summary:
The CacheLib getdeps_linux GitHub Actions CI build failed due to a flaky test timeout. Test navy-test-BlockCacheTest.BlockCache.ExpiredItemDestructorCallback exceeded the 120-second CTest limit by 0.09 seconds. This is an intermittent timing issue on shared CI runners where busy-wait loops using std::this_thread::yield() get CPU-starved.

Increase the global CTest per-test timeout from 120s to 300s to provide adequate headroom for CI variability. This fix was validated on GitHub Actions CI: https://github.com/facebook/CacheLib/actions/runs/24465951879
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overriding_review_checks_triggers_an_audit_and_retroactive_review
Oncall Short Name: cachelib

Differential Revision: D101006545

fbshipit-source-id: 5ec3728b4b7dbea38d49c5fbea5a600482d04644

Author: rlyerly

cachelib/CMakeLists.txt

@@ -166,7 +166,7 @@ function (generic_add_source_test TEST_PREFIX SOURCE_FILE)
                         TEST_PREFIX "${TEST_PREFIX}-${TEST_NAME}."
                         TEST_LIST "test_cases")
   # use same timeout as folly
-  set_tests_properties(${test_cases} PROPERTIES TIMEOUT 120)
+  set_tests_properties(${test_cases} PROPERTIES TIMEOUT 300)
 endfunction()