burn_tests: ensure burntest does not block pytest

pytest discovers test_*.py and *_test.py, so it should be hidden now
in addition, the code should only run when the script is run explicitly,
not when imported

trezorlib/tests/burn_tests/burntest.py

@@ -37,6 +37,7 @@ def get_device():
         raise RuntimeError("No debuggable device found")
 
 
+if __name__ == "__main__":
     wirelink = get_device()
     client = TrezorClientDebugLink(wirelink)
     client.open()