tests/micropython: Make uio-using tests skippable.

tests/micropython/heapalloc_traceback.py

@@ -2,7 +2,12 @@
 
 import micropython
 import sys
-import uio
+try:
+    import uio
+except ImportError:
+    import sys
+    print("SKIP")
+    sys.exit()
 
 # preallocate exception instance with some room for a traceback
 global_exc = StopIteration()