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/zerosized.cpp

@@ -81,7 +81,7 @@ template<typename VectorType> void zeroSizedVector()
   }
 }
 
-void test_zerosized()
+EIGEN_DECLARE_TEST(zerosized)
 {
   zeroSizedMatrix<Matrix2d>();
   zeroSizedMatrix<Matrix3i>();