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/numext.cpp

@@ -33,7 +33,7 @@ void check_abs() {
   }
 }
 
-void test_numext() {
+EIGEN_DECLARE_TEST(numext) {
   CALL_SUBTEST( check_abs<bool>() );
   CALL_SUBTEST( check_abs<signed char>() );
   CALL_SUBTEST( check_abs<unsigned char>() );