Get rid of EIGEN_TEST_FUNC, unit tests must now be declared with EIGEN_DECLARE_TEST(mytest) { /* code */ }.

This provide several advantages:
- more flexibility in designing unit tests
- unit tests can be glued to speed up compilation
- unit tests are compiled with same predefined macros, which is a requirement for zapcc

test/ctorleak.cpp

@@ -33,7 +33,7 @@ Index Foo::object_limit = 0;
 #undef EIGEN_TEST_MAX_SIZE
 #define EIGEN_TEST_MAX_SIZE 3
 
-void test_ctorleak()
+EIGEN_DECLARE_TEST(ctorleak)
 {
   typedef Matrix<Foo, Dynamic, Dynamic> MatrixX;
   typedef Matrix<Foo, Dynamic, 1> VectorX;