Add FileTest::size method

I don't see any reason to leave out this method, the code I'm trying to test is using it and I'm having troubles covering this code with tests.

To be very specific, I'm trying to test Homebrew Cask code which then calls Homebrew itself and ends up here:
https://github.com/Homebrew/homebrew/blob/bc0c203934f67624a4a3cd2bd15f1b4eb93b0236/Library/Homebrew/download_strategy.rb#L242-244

on following error:
```
NoMethodError:
       undefined method `size?' for FakeFS::FileTest:Module
```

Tests will fail with the code currently on `HEAD`, but pass after merging #268