Edited test runner to allow exercise skeletons.
Edited test runner to allow exercise skeletons.

Some more complicated exercises come with a skeleton in "src/lib.rs", but the
test runner would fail if "src" is already present in an exercise's directory.

medek/xrust,petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust,petertseng/exercism-rust,sacherjj/rust,exercism/xrust,spazm/xrust,ekse/exercism-rust,Jeff-Hostetler/xrust,spazm/xrust,stevejb71/xrust,sacherjj/rust,stevejb71/xrust,sacherjj/rust,medek/xrust,pminten/xrust,Jeff-Hostetler/xrust,pminten/xrust,ekse/exercism-rust