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/fastmath.cpp

@@ -88,7 +88,7 @@ void check_inf_nan(bool dryrun) {
   }
 }
 
-void test_fastmath() {
+EIGEN_DECLARE_TEST(fastmath) {
   std::cout << "*** float *** \n\n"; check_inf_nan<float>(true);
   std::cout << "*** double ***\n\n"; check_inf_nan<double>(true);
   std::cout << "*** long double *** \n\n"; check_inf_nan<long double>(true);