test: change service account to oauth client

[xoiga123]

I got hit with

The user's Drive storage quota has been exceeded

when trying to test with a service account created from my gmail.com account. Apparently service accounts have 0 Drive storage space available, and I couldn't do any test with client.create(name), since that requires creating a sheet in the service account's Drive.

I tried some workarounds to no avail:

1. Share a folder with the service account with Editor permission and do client.create(name, folder_id=...), still fails.
2. Enable domain-wide-delegation so that the service account can impersonate my gmail.com account, fails because I cannot login to admin.google.com with a gmail.com account.

Unless I'm seriously misunderstanding something, the only way for a regular gmail.com user to add new tests now is by authenticating with OAuth instead so that test sheets can be created.

[alifeee]

sorry, I do not have much time to review PRs right now. I hope that is okay :] — I am discussing gspread maintenance with @lavigne958.