Disable object stack allocation verification under GCStress.

Disable object stack allocation verification under GCStress.

ObjectStackAllocationTests use GC.GetAllocatedBytesForCurrentThread to
verify object stack allocations. Under GCStress the vm may initiate additional
heap allocations in GCHeap::StressHeap (see the call to 'pGenGCHeap->allocate' below).

This change re-enables ObjectStackAllocationTests and updates then to not verify stack allocations under GCStress.
It's useful to run the tests even without the verification to catch crashes, gc asserts, etc.

```
if (Interlocked::Increment(&OneAtATime) == 0 &&
        !TrackAllocations()) // Messing with object sizes can confuse the profiler (see ICorProfilerInfo::GetObjectSize)
    {
        StringObject* str;

        // If the current string is used up
        if (HndFetchHandle(m_StressObjs[m_CurStressObj]) == 0)
        {
            // Populate handles with strings
            int i = m_CurStressObj;
            while(HndFetchHandle(m_StressObjs[i]) == 0)
            {
                _ASSERTE(m_StressObjs[i] != 0);
                unsigned strLen = (LARGE_OBJECT_SIZE - 32) / sizeof(WCHAR);
                unsigned strSize = PtrAlign(StringObject::GetSize(strLen));

                // update the cached type handle before allocating
                SetTypeHandleOnThreadForAlloc(TypeHandle(g_pStringClass));
                str = (StringObject*) pGenGCHeap->allocate (strSize, acontext);
                if (str)
                {
                    str->SetMethodTable (g_pStringClass);
                    str->SetStringLength (strLen);

                    HndAssignHandle(m_StressObjs[i], ObjectToOBJECTREF(str));
                }
                i = (i + 1) % NUM_HEAP_STRESS_OBJS;
                if (i == m_CurStressObj) break;
            }

            // advance the current handle to the next string
            m_CurStressObj = (m_CurStressObj + 1) % NUM_HEAP_STRESS_OBJS;
        }

        // Get the current string
        str = (StringObject*) OBJECTREFToObject(HndFetchHandle(m_StressObjs[m_CurStressObj]));
        if (str)
        {
            // Chop off the end of the string and form a new object out of it.
            // This will 'free' an object at the begining of the heap, which will
            // force data movement.  Note that we can only do this so many times.
            // before we have to move on to the next string.
            unsigned sizeOfNewObj = (unsigned)Align(min_obj_size * 31);
            if (str->GetStringLength() > sizeOfNewObj / sizeof(WCHAR))
            {
                unsigned sizeToNextObj = (unsigned)Align(size(str));
                uint8_t* freeObj = ((uint8_t*) str) + sizeToNextObj - sizeOfNewObj;
                pGenGCHeap->make_unused_array (freeObj, sizeOfNewObj);
                str->SetStringLength(str->GetStringLength() - (sizeOfNewObj / sizeof(WCHAR)));
            }
            else
            {
                // Let the string itself become garbage.
                // will be realloced next time around
                HndAssignHandle(m_StressObjs[m_CurStressObj], 0);
            }
        }
    }
    Interlocked::Decrement(&OneAtATime);
```

Commit migrated from https://github.com/dotnet/coreclr/commit/5ef00810b53dcb7cbc4f2cb152ca6af971284e82