[tests] Fix some unit tests random fail issue (#1966)

src/core/meshcop/dataset.cpp

@@ -52,6 +52,7 @@ Dataset::Dataset(const Tlv::Type aType) :
     mLength(0),
     mType(aType)
 {
+    memset(mTlvs, 0, sizeof(mTlvs));
 }
 
 void Dataset::Clear(void)

tests/unit/test_platform.cpp

@@ -495,7 +495,7 @@ exit:
         (void)aValue;
         (void)aValueLength;
 
-        return OT_ERROR_NONE;
+        return OT_ERROR_NOT_FOUND;
     }
 
     otError otPlatSettingsSet(otInstance *aInstance, uint16_t aKey, const uint8_t *aValue, uint16_t aValueLength)