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/gpu_basic.cu

@@ -15,7 +15,6 @@
 
 #define EIGEN_TEST_NO_LONGDOUBLE
 #define EIGEN_TEST_NO_COMPLEX
-#define EIGEN_TEST_FUNC gpu_basic
 #define EIGEN_DEFAULT_DENSE_INDEX_TYPE int
 
 #include "main.h"
@@ -162,7 +161,7 @@ struct matrix_inverse {
   }
 };
 
-void test_gpu_basic()
+EIGEN_DECLARE_TEST(gpu_basic)
 {
   ei_test_init_gpu();