Fix merge-nunit-xml issue

[AnatolyPristensky]

Fix merge-nunit-xml issue

Related issue: 1208

[AlexeyBudynkov]

This PR has been tested.
Output before:
...
Current test status: Done!
Total scenarios: 13
13 passed
0 failed
Total steps: 28

Downloaded artifacts to /Users/PC/Downloads/Debug/nunit_xml_zip.zip
Error: The "data" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received an instance of Document

Further error details: For help, please send both the reported error above and the following environment information to us by going to https://appcenter.ms/apps and opening a support ticket (using the help menu in the upper right corner of the screen)

Environment: darwin
App Upload Id: konstantin-bobkov/Horoscope
Timestamp: 1643201964046
Operation: RunUITestsCommand
Exit Code: 3
User Email: v-abudynkov@microsoft.com
User Name: v-abudynkov-microsoft.com
User Id: 465cdc15-e141-48c1-bc16-333e57fa382f

Output after:
...
Current test status: Done!
Total scenarios: 13
13 passed
0 failed
Total steps: 28

Downloaded artifacts to /Users/PC/Downloads/Debug/nunit_xml_zip.zip

Test Report: https://appcenter.ms/users/konstantin-bobkov/apps/Horoscope/test/series/launch-tests/runs/572a2cf1-2fc9-469e-a74f-358fd942fc52

Also an XML-file has written locally.