strict_not_null for unique_ptr (#1179)

- `strict_not_null<std::unique_ptr<int>>{ std::make_unique<int>()}` failed to compile
  - `strict_not_null` ctor needs to move the passed `unique_ptr`, not copy
  - Copied `not_null` `TestNotNullConstructors` for `strict_not_null`
- The `noexcept` specifiers on the `strict_not_null` and `not_null` constructors were out of sync.
- Added unit test for `not_null<unique_ptr<T>>` and for `strict_not_null<unique_ptr<T>>`
- Added unit test for `gsl::swap` for two `strict_not_null`
- Added unit test for `gsl::swap` for `not_null` and `strict_not_null`

Co-authored-by: Werner Henze <w.henze@avm.de>

tests/notnull_tests.cpp

@@ -178,6 +178,14 @@ TEST(notnull_tests, TestNotNullConstructors)
         EXPECT_DEATH((not_null<decltype(pi)>(pi)), expected);
     }
 
+    {
+        // from unique pointer
+        not_null<std::unique_ptr<int>> x(
+            std::make_unique<int>(10)); // unique_ptr<int> is nullptr assignable
+
+        EXPECT_DEATH((not_null<std::unique_ptr<int>>(std::unique_ptr<int>{})), expected);
+    }
+
     {
         // from pointer to local
         int t = 42;