Mark new Gallery test as non-flaky (#56700)
Mark new Gallery test as non-flaky (#56700)

The test has been consistently green for the last few commits.
Hixie/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter