Reduce flakiness of memory_store_test (#318)

After looking at CI test failures it seems that the ASAN builds cause an
RSS increase of ~14%  to ~17%. Set the limit to 20% to reduce flakiness.

cas/store/tests/memory_store_test.rs

@@ -78,9 +78,9 @@ mod memory_store_tests {
     // Regression test for: https://github.com/TraceMachina/turbo-cache/issues/289.
     #[tokio::test]
     async fn ensure_full_copy_of_bytes_is_made_test() -> Result<(), Error> {
-        // Arbitrary value, this may be increased if we find out that this is too low
-        // for some kernels/operating systems.
-        const MAXIMUM_MEMORY_USAGE_INCREASE_PERC: f64 = 1.1; // 10% increase.
+        // Arbitrary value, this may be increased if we find out that this is
+        // too low for some kernels/operating systems.
+        const MAXIMUM_MEMORY_USAGE_INCREASE_PERC: f64 = 1.2; // 20% increase.
 
         let store_owned = MemoryStore::new(&config::stores::MemoryStore::default());
         let store = Pin::new(&store_owned);