build: Start running libcore tests

src/libcore/core.rs

@@ -25,4 +25,11 @@ mod core {
     const warn : u32 = 1_u32;
     const info : u32 = 2_u32;
     const debug : u32 = 3_u32;
+}
+
+// Similar to above. Some magic to make core testable.
+#[cfg(test)]
+mod std {
+    use std;
+    import std::test;
 }