Fix for for setting test-threads to 0
Fix for for setting test-threads to 0

Running test with cargo test -- --test-threads=0 causes cargo to
hang as 0 is a valid usize. Adding zero threads as a special case
to the error handling.

GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust