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_uint128.cpp

@@ -144,7 +144,7 @@ void test_misc2() {
 #endif
 
 
-void test_cxx11_tensor_uint128()
+EIGEN_DECLARE_TEST(cxx11_tensor_uint128)
 {
 #ifdef EIGEN_NO_INT128
   // Skip the test on compilers that don't support 128bit integers natively