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

unsupported/test/cxx11_tensor_of_strings.cpp

@@ -141,7 +141,7 @@ static void test_initialization()
 }
 
 
-void test_cxx11_tensor_of_strings()
+EIGEN_DECLARE_TEST(cxx11_tensor_of_strings)
 {
   // Beware: none of this is likely to ever work on a GPU.
   CALL_SUBTEST(test_assign());