Add tests for from_bits.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>

src/libstd/bitflags.rs

@@ -207,6 +207,13 @@ mod tests {
         assert_eq!(FlagABC.bits(), 0x00000111);
     }
 
+    #[test]
+    fn test_from_bits() {
+        assert!(unsafe { Flags::from_bits(0x00000000) } == Flags::empty());
+        assert!(unsafe { Flags::from_bits(0x00000001) } == FlagA);
+        assert!(unsafe { Flags::from_bits(0x00000111) } == FlagABC);
+    }
+
     #[test]
     fn test_is_empty(){
         assert!(Flags::empty().is_empty());