Add test for FS::open("w+") (#7241)
Add test for FS::open("w+") (#7241)

Verify that a file is truncated on opening with w+ even if no data is
written to it, for all FSes.

Co-authored-by: Develo <f1eea6592757f97fa52d722d4289075385d7ae74@gmail.com>
sticilface/Arduino,sticilface/Arduino,esp8266/Arduino,esp8266/Arduino,sticilface/Arduino,esp8266/Arduino,sticilface/Arduino,esp8266/Arduino,esp8266/Arduino,sticilface/Arduino