tests/basics/gc1: Garbage collector threshold() coverage.

tests/basics/gc1.py

@@ -20,3 +20,11 @@ if hasattr(gc, 'mem_free'):
     # just test they execute and return an int
     assert type(gc.mem_free()) is int
     assert type(gc.mem_alloc()) is int
+
+if hasattr(gc, 'threshold'):
+    # uPy has this extra function
+    # check execution and returns
+    assert(gc.threshold(1) is None)
+    assert(gc.threshold() == 0)
+    assert(gc.threshold(-1) is None)
+    assert(gc.threshold() == -1)