Fix VS2013 compiler error in Entity_test.cc

Might be a weird compiler bug.
The compiler was throwing errors at me 
```
  Entity_test.cc
..\entityx\Entity_test.cc(361): error C2327: 'EntityManagerFixture::ev' : is not a type name, static, or enumerator
..\entityx\Entity_test.cc(361): error C2065: 'ev' : undeclared identifier
..\entityx\Entity_test.cc(361): error C2228: left of '.subscribe' must have class/struct/union
          type is 'unknown-type'
..\entityx\Entity_test.cc(362): error C2327: 'EntityManagerFixture::ev' : is not a type name, static, or enumerator
..\entityx\Entity_test.cc(362): error C2065: 'ev' : undeclared identifier
..\entityx\Entity_test.cc(362): error C2228: left of '.subscribe' must have class/struct/union
          type is 'unknown-type'
..\entityx\Entity_test.cc(370): error C2327: 'EntityManagerFixture::em' : is not a type name, static, or enumerator
..\entityx\Entity_test.cc(370): error C2065: 'em' : undeclared identifier
..\entityx\Entity_test.cc(370): error C2228: left of '.create' must have class/struct/union
          type is 'unknown-type'
```

Moving the `struct ComponentAddedEventReceiver` outside the test function or removing the `struct ComponentAddedEventReceiver`'s member default value fix it.