Add testing to screenshot and printDetails method

[jonahwilliams]

Description

Note: Apologies for large diff due to removal of unnecessary group.

To facilitate the eventual refactor of resident_runner, it must be thoroughly unit tested. In this PR I attempted to add test cases for the invariants of the screenshot method, adding documentation, as well as a rough smoke test of print help.

[codecov]

Codecov Report

Merging #36418 into master will increase coverage by 1.59%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #36418      +/-   ##
==========================================
+ Coverage   54.03%   55.63%   +1.59%     
==========================================
  Files         188      188              
  Lines       17344    17346       +2     
==========================================
+ Hits         9372     9650     +278     
+ Misses       7972     7696     -276

Flag	Coverage Δ
#flutter_tool	55.63% <100%> (+1.59%)	⬆️

Impacted Files	Coverage Δ
...ackages/flutter_tools/lib/src/resident_runner.dart	45.18% <100%> (+7.55%)	⬆️
...ckages/flutter_tools/lib/src/ios/code_signing.dart	96.87% <0%> (-1.57%)	⬇️
packages/flutter_tools/lib/src/version.dart	96.05% <0%> (-1.48%)	⬇️
packages/flutter_tools/lib/src/build_info.dart	69.61% <0%> (-1.11%)	⬇️
...s/flutter_tools/lib/src/test/flutter_platform.dart	28.64% <0%> (-0.28%)	⬇️
.../flutter_tools/lib/src/android/android_device.dart	31.37% <0%> (-0.26%)	⬇️
packages/flutter_tools/lib/src/vmservice.dart	39.13% <0%> (+0.31%)	⬆️
...ges/flutter_tools/lib/src/application_package.dart	66.54% <0%> (+1.12%)	⬆️
packages/flutter_tools/lib/src/base/process.dart	82.81% <0%> (+1.56%)	⬆️
packages/flutter_tools/lib/src/run_hot.dart	48.68% <0%> (+2.85%)	⬆️
... and 4 more

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[christopherfujino]

LGTM