tests/micropython: Add tests for heap_lock, and emergency exceptions.

tests/micropython/emg_exc.py

+# test that emergency exceptions work
+
+import micropython
+import sys
+
+# some ports need to allocate heap for the emg exc
+try:
+    micropython.alloc_emergency_exception_buf(256)
+except AttributeError:
+    pass
+
+def f():
+    micropython.heap_lock()
+    try:
+        raise ValueError(1)
+    except ValueError as er:
+        sys.print_exception(er)
+    micropython.heap_unlock()
+
+f()

tests/micropython/emg_exc.py.exp

+ValueError: 

tests/micropython/heap_lock.py

+# check that heap_lock/heap_unlock work as expected
+
+import micropython
+
+micropython.heap_lock()
+
+try:
+    print([])
+except MemoryError:
+    print('MemoryError')
+
+micropython.heap_unlock()
+
+print([])

tests/micropython/heap_lock.py.exp

+MemoryError
+[]

tests/micropython/heapalloc.py

 # check that we can do certain things without allocating heap memory
 
-import gc
+import micropython
 
 def f1(a):
     print(a)
@@ -28,12 +28,7 @@ def test():
         f2(i, i)        # 2 args
     f3(1, 2, 3, 4)  # function with lots of local state
 
-# call h with heap allocation disabled and all memory used up
-gc.disable()
-try:
-    while True:
-        'a'.lower # allocates 1 cell for boundmeth
-except MemoryError:
-    pass
+# call test() with heap allocation disabled
+micropython.heap_lock()
 test()
-gc.enable()
+micropython.heap_unlock()