Test heap and stack - warning - does not iterate

The test is outdated, as it assumes the stack is above the heap (and smaller than heap).

frdm-k64f-gcc output:

```
{{timeout;10}}
{{host_test_name;default_auto}}
{{description;Heap & Stack}}
{{test_id;MBED_13}}
{{start}}
Initial stack/heap geometry:
stack pointer:V 0x1fffa047
heap pointer :^ 0x1fffacb0

stack pointer: 0x1fffa02f
heap pointer: 0x1fffadc0

[WARNING] The Stack/Heap collision was not detected

Allocated (0)Kb in (1) iterations

{{success}}
{{end}}
```

This test should be in ualloc, and should be corrected/improved.

Thanks for bringing this up to @akselsm

Locate the heap and stack test and inspect the ualloc area, starting with how the test measures stack and heap geometry. Compare its assumptions with the frdm-k64f-gcc output and determine what corrected behavior should be covered; done means the test no longer reports a false warning and iterates as intended.

Written by the indexing model from the issue text.