Cleanup self-include tests to compile solely as object files

test/unordered/self_include_tests_obj.cpp

@@ -1,5 +1,5 @@
 #ifndef BOOST_UNORDERED_HEADER
-#error "this requires a class template be passed as a macro"
+#error "this test requires a class template be passed as a macro"
 #endif
 
 #define STRINGIZE(text) STRINGIZE_I(text)
@@ -7,5 +7,3 @@
 
 #define HEADER STRINGIZE(BOOST_UNORDERED_HEADER)
 #include HEADER
-
-int main() {}