testsuite: Make main fn public

Author: catamorphism

src/test/run-pass/issue-4107.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-fn main() {
+pub fn main() {
     let id: &Mat2<float> = &Matrix::identity();
 }