[Python] Reset chip error in test commissioner

Make sure to reset the chip error in test commissioner on reset. This
avoid spurious errors. The Python side reads the error as soon as
mTestCommissionerUsed is set, which happens unconditionally. Hence
clearing the error is necessary.

Author: agners

src/controller/python/OpCredsBinding.cpp

@@ -259,6 +259,7 @@ class TestCommissioner : public chip::Controller::AutoCommissioner
         mPrematureCompleteAfter = chip::Controller::CommissioningStage::kError;
         mReadCommissioningInfo  = chip::Controller::ReadCommissioningInfo();
         mNeedsDST               = false;
+        mCompletionError        = CHIP_NO_ERROR;
     }
     bool GetTestCommissionerUsed() { return mTestCommissionerUsed; }
     void OnCommissioningSuccess(chip::PeerId peerId) { mReceivedCommissioningSuccess = true; }