tests/extmod/asyncio_heaplock.py: Improve skip detection of test.

Signed-off-by: Damien George <damien@micropython.org>

tests/extmod/asyncio_heaplock.py

@@ -4,7 +4,11 @@
 # - StreamWriter.write, stream is blocked and data to write is a bytes object
 # - StreamWriter.write, when stream is not blocked
 
-import micropython
+try:
+    import asyncio, micropython
+except ImportError:
+    print("SKIP")
+    raise SystemExit
 
 # strict stackless builds can't call functions without allocating a frame on the heap
 try:
@@ -24,12 +28,6 @@ except RuntimeError:
     print("SKIP")
     raise SystemExit
 
-try:
-    import asyncio
-except ImportError:
-    print("SKIP")
-    raise SystemExit
-
 
 class TestStream:
     def __init__(self, blocked):