fix(test): Fix the flaky "learn more" SMS test. (#4881) r=@jrgm
fix(test): Fix the flaky "learn more" SMS test. (#4881) r=@jrgm

Sometimes the "_learn_more" window would not be open, esp when run
on slow machines. Use the `switchToWindow` helper which will check
for open windows, and use the window index instead of name.

fixes #4880
mozilla/fxa-content-server,shane-tomlinson/fxa-content-server,mozilla/fxa-content-server,mozilla/fxa-content-server,shane-tomlinson/fxa-content-server,shane-tomlinson/fxa-content-server