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/array_of_string.cpp

@@ -9,7 +9,7 @@
 
 #include "main.h"
 
-void test_array_of_string()
+EIGEN_DECLARE_TEST(array_of_string)
 {
   typedef Array<std::string,1,Dynamic> ArrayXs;
   ArrayXs a1(3), a2(3), a3(3), a3ref(3);