Revert "test: Remove swappable-test" due to test failures

This reverts commit aabf84cdd8.

src/test/run-pass/swappable-test.rs

@@ -0,0 +1,13 @@
+import swappable::{swappable, methods};
+
+fn main() {
+    let d = swappable(3);
+    assert d.get() == 3;
+    d.set(4);
+    assert d.get() == 4;
+    d.swap { |i| i + 1 };
+    assert d.get() == 5;
+    assert d.with { |i| i + 1 } == 6;
+    assert d.get() == 5;
+    assert swappable::unwrap(d) == 5;
+}