Turn on scenario app screenshots

[jmagman]

Save a recording/screenshot to the uploaded xcresult when a Scenario test fails.

No more errant spew as reported in #19668 when this was originally turned off.

 Test Case '-[StatusBarTest testTapStatusBar]' started.
     t =     0.00s Start Test at 2023-12-14 18:20:34.207
...
     t =     2.69s     Synthesize event
     t =     2.70s         Find the StatusBar at {{0.0, 0.0}, {375.0, 20.0}}
     t =     2.72s         Find the StatusBar at {{0.0, 0.0}, {375.0, 20.0}}
     t =     2.74s         Find the StatusBar at {{0.0, 0.0}, {375.0, 20.0}}
     t =     3.03s     Wait for com.apple.springboard to idle
     t =     3.43s Waiting 1.0s for "0,PointerChange.add,device=0,buttons=0,signalKind=PointerSignalKind.none" TextField to exist
     t =     4.43s     Checking `Expect predicate `exists == 1` for object "0,PointerChange.add,device=0,buttons=0,signalKind=PointerSignalKind.none" TextField`
     t =     4.43s         Checking existence of `"0,PointerChange.add,device=0,buttons=0,signalKind=PointerSignalKind.none" TextField`
     t =     4.45s         Capturing element debug description
     t =     4.45s Checking existence of `"0,PointerChange.add,device=0,buttons=0,signalKind=PointerSignalKind.none" TextField`
 /Volumes/Work/s/w/ir/cache/builder/src/out/ios_debug_sim/scenario_app/Scenarios/ScenariosUITests/StatusBarTest.m:37: error: -[StatusBarTest testTapStatusBar] : ((exists) is true) failed
     t =     4.48s Tear Down

https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8761683198070320113/+/u/test:_Scenario_App_Integration_Tests__3_/stdout

Fixes flutter/flutter#140192

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Changes reported for pull request #49066 at sha c756ff3

[vashworth]

LGTM

[jmagman]

It looks like this pull request may not have tests.

I got distracted yak shaving flutter/cocoon#3358

[stuartmorgan-g]

test-exempt: is a test