Test fallback file implementation on Linux/Android

[josephlr]

Fixes #289 for Linux. Depends on #471

Now when running cargo test on Linux, we see additional tests for the file-based implementation:

running 11 tests
test error::tests::test_size ... ok
test imp::fallback::fill_large ... ok
test imp::fallback::fill_zero ... ok
test tests::fill_large ... ok
test tests::fill_zero ... ok
test imp::fallback::multithreading ... ok
test tests::multithreading ... ok
test imp::fallback::fill_small ... ok
test tests::fill_small ... ok
test imp::fallback::fill_huge ... ok
test tests::fill_huge ... ok

[josephlr]

Closing to keep conversation in #471

Nevermind, this change should probably be review separately.