tests/extmod: Improve skip detection of extmod tests.

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

tests/extmod/tls_threads.py

@@ -1,6 +1,5 @@
 # Ensure that SSL sockets can be allocated from multiple
 # threads without thread safety issues
-import unittest
 
 try:
     import _thread
@@ -11,6 +10,8 @@ except ImportError:
     print("SKIP")
     raise SystemExit
 
+import unittest
+
 
 class TestSocket(io.IOBase):
     def write(self, buf):