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/autodiff.cpp

@@ -354,7 +354,7 @@ double bug_1264() {
 
 #endif
 
-void test_autodiff()
+EIGEN_DECLARE_TEST(autodiff)
 {
   for(int i = 0; i < g_repeat; i++) {
     CALL_SUBTEST_1( test_autodiff_scalar<1>() );