Attempt to fix flaky test re: flashing highlight decorations
Attempt to fix flaky test re: flashing highlight decorations

Based on the assertion failures seen in
https://github.com/atom/atom/issues/15158#issue-247808059, it seems that
the flash for class 'c' sometimes ends before the flash for class 'd'
happens. Prior to this change, we only flashed class 'c' for 100ms, and
perhaps that isn't always enough time.

In this commit, we increase the flash duration from 100ms to 1000ms,
greatly increasing the likelihood that we're allowing enough time for
the flash on class 'd' to take place before the flash for class 'c'
ends. We also extract the scenario into its own test, so that 1) we can
more clearly explain the scenario that these assertions are testing and
2) future intermittent test failures will be easier to isolate.

andrewleverette/atom,liuderchi/atom,kevinrenaers/atom,decaffeinate-examples/atom,ardeshirj/atom,atom/atom,kevinrenaers/atom,brettle/atom,liuderchi/atom,brettle/atom,liuderchi/atom,atom/atom,Mokolea/atom,FIT-CSE2410-A-Bombs/atom,ardeshirj/atom,ardeshirj/atom,stinsonga/atom,t9md/atom,FIT-CSE2410-A-Bombs/atom,atom/atom,decaffeinate-examples/atom,stinsonga/atom,t9md/atom,PKRoma/atom,PKRoma/atom,t9md/atom,decaffeinate-examples/atom,liuderchi/atom,Mokolea/atom,Arcanemagus/atom,kevinrenaers/atom,decaffeinate-examples/atom,FIT-CSE2410-A-Bombs/atom,andrewleverette/atom,PKRoma/atom,Mokolea/atom,brettle/atom,stinsonga/atom,stinsonga/atom,Arcanemagus/atom,Arcanemagus/atom,andrewleverette/atom