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/packetmath.cpp

@@ -627,7 +627,7 @@ template<typename Scalar> void packetmath_scatter_gather()
   }
 }
 
-void test_packetmath()
+EIGEN_DECLARE_TEST(packetmath)
 {
   for(int i = 0; i < g_repeat; i++) {
     CALL_SUBTEST_1( packetmath<float>() );