The main motivation is to move toward running those tests for a specific
target, that is, if a test won't run on Windows, then we shouldn't build
it. This was previously the default behavior and prevented a successful
run on AppVeyor.
I borrowed this pattern from the tests in the Cargo project.
Makes `parse_size` return a `Result` where the `Err` part indicates whether
there was a parsing error, or the parse size is too big to store. Also makes the
value parsed a `u64` rather than a `usize`.
Adds unit tests for `parse_size` and integration tests using the suffix
multiplier in a number passed with the `-n` flag.
