tests/extmod/random_extra_float.py: Skip when funcs not available.

This test was factored out from `random_extra.py` back in commit
6572029dc0, and the skip logic copied from
that file.  But the skip logic needs to test that the `random` and
`uniform` functions exist, not `randint`.

This commit fixes that skip logic.

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

tests/extmod/random_extra_float.py

@@ -1,12 +1,8 @@
 try:
     import random
-except ImportError:
-    print("SKIP")
-    raise SystemExit
 
-try:
+    random.random
-    random.randint
+except (ImportError, AttributeError):
-except AttributeError:
     print("SKIP")
     raise SystemExit