Refactor tests to use `MemoryFileSystem` for tests that depend on time

[eliasyishak]

Currently, we are using the MemoryFileSystem.test() constructor for all tests. This uses a fake clock internally which works great for tests that may fail due to race conditions.

However, all tests don't require a fake clock, as pointed out here, some may actually benefit from using the real clock.

We should instead use the appropriate constructor of MemoryFileSystem on a test-by-test case instead of globally setting it up in setUp