test_limit_reference: Avoid virtual-move-assign warning

"= delete" Bases's move operations because the defaulted one
would call non-trivial assignment operators in the virtual bases.

This avoids this compiler warning, at least with g++ 9.2:

test_limit_reference.cc:12:7: error: defaulted move assignment for ‘{anonymous}::Base’ calls a non-trivial move assignment operator for virtual base ‘sigc::trackable’ [-Werror=virtual-move-assign]
   12 | class Base : virtual public sigc::trackable
      |       ^~~~
test_limit_reference.cc:22:7: error: defaulted move assignment for ‘{anonymous}::Derived’ calls a non-trivial move assignment operator for virtual base ‘{anonymous}::Base’ [-Werror=virtual-move-assign]
   22 | class Derived : virtual public Base, public Base2

It would be nice if this test had a comment saying what it is testing.

1 files changed, 5 insertions, 0 deletions

diff --git a/tests/test_limit_reference.cc b/tests/test_limit_reference.cc
index 5a7a8aa..8d1340d 100644
--- a/tests/test_limit_reference.cc
+++ b/tests/test_limit_reference.cc
@@ -11,6 +11,11 @@ std::ostringstream result_stream;
 
 class Base : virtual public sigc::trackable
 {
+public:
+  Base() {}
+
+  Base(Base&& src) = delete;
+  Base& operator=(Base&& src) = delete;
 };
 
 class Base2