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/NumericalDiff.cpp

@@ -107,7 +107,7 @@ void test_central()
     VERIFY_IS_APPROX(jac, actual_jac);
 }
 
-void test_NumericalDiff()
+EIGEN_DECLARE_TEST(NumericalDiff)
 {
     CALL_SUBTEST(test_forward());
     CALL_SUBTEST(test_central());