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/sizeoverflow.cpp

@@ -34,7 +34,7 @@ void triggerVectorBadAlloc(Index size)
   VERIFY_THROWS_BADALLOC( VectorType v; v.conservativeResize(size) );
 }
 
-void test_sizeoverflow()
+EIGEN_DECLARE_TEST(sizeoverflow)
 {
   // there are 2 levels of overflow checking. first in PlainObjectBase.h we check for overflow in rows*cols computations.
   // this is tested in tests of the form times_itself_gives_0 * times_itself_gives_0