Rename test modules to "tests" for consistency.

The majority of the test modules were already named "tests", and that's
what the example in the Rust book uses, so switch to that for all test
modules, for consistency.

lib/codegen/src/regalloc/virtregs.rs

@@ -396,7 +396,7 @@ impl VirtRegs {
 }
 
 #[cfg(test)]
-mod test {
+mod tests {
     use super::*;
     use entity::EntityRef;
     use ir::Value;