Bring tests back into rustc source tarball

They were missing after recent move from src/test to tests.

Author: tmiasko

src/bootstrap/dist.rs

@@ -962,7 +962,7 @@ impl Step for PlainSourceTarball {
             "Cargo.toml",
             "Cargo.lock",
         ];
-        let src_dirs = ["src", "compiler", "library"];
+        let src_dirs = ["src", "compiler", "library", "tests"];
 
         copy_src_dirs(builder, &builder.src, &src_dirs, &[], &plain_dst_src);