Add -Wdeprecated to test suite and fix associated warnings (#1191)

This commit turns on `-Wdeprecated` in the test suite and fixes several
associated deprecation warnings that show up as a result:

- in C++17 `static constexpr` members are implicitly inline; our
  redeclaration (needed for C++11/14) is deprecated in C++17.

- various test suite classes have destructors and rely on implicit copy
  constructors, but implicit copy constructor definitions when a
  user-declared destructor is present was deprecated in C++11.

- Eigen also has various implicit copy constructors, so just disable
  `-Wdeprecated` in `eigen.h`.

tests/test_class.cpp

@@ -14,6 +14,9 @@
 TEST_SUBMODULE(class_, m) {
     // test_instance
     struct NoConstructor {
+        NoConstructor() = default;
+        NoConstructor(const NoConstructor &) = default;
+        NoConstructor(NoConstructor &&) = default;
         static NoConstructor *new_instance() {
             auto *ptr = new NoConstructor();
             print_created(ptr, "via new_instance");
@@ -82,7 +85,12 @@ TEST_SUBMODULE(class_, m) {
     m.def("dog_bark", [](const Dog &dog) { return dog.bark(); });
 
     // test_automatic_upcasting
-    struct BaseClass { virtual ~BaseClass() {} };
+    struct BaseClass {
+        BaseClass() = default;
+        BaseClass(const BaseClass &) = default;
+        BaseClass(BaseClass &&) = default;
+        virtual ~BaseClass() {}
+    };
     struct DerivedClass1 : BaseClass { };
     struct DerivedClass2 : BaseClass { };