Add most of the widget tests; add more web test shards

[yjbanov]

Description

Add most of the /test/widgets/ tests to the CI matrix and increase the number of Web test shards.

Related Issues

Fixes #41913

[ferhatb]

should have diff number

[yjbanov]

Done.

[ferhatb]

Please add comment about id

[yjbanov]

Done.

[codecov]

Codecov Report

Merging #42807 into master will decrease coverage by 0.91%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #42807      +/-   ##
==========================================
- Coverage   60.31%   59.39%   -0.92%     
==========================================
  Files         194      194              
  Lines       18920    18910      -10     
==========================================
- Hits        11412    11232     -180     
- Misses       7508     7678     +170

Flag	Coverage Δ
#flutter_tool	59.39% <ø> (-0.92%)	⬇️

Impacted Files	Coverage Δ
packages/flutter_tools/lib/src/emulator.dart	9.48% <0%> (-61.21%)	⬇️
...kages/flutter_tools/lib/src/ios/ios_emulators.dart	8.69% <0%> (-39.14%)	⬇️
packages/flutter_tools/lib/src/run_hot.dart	38.44% <0%> (-30.03%)	⬇️
...ackages/flutter_tools/lib/src/resident_runner.dart	37.37% <0%> (-20%)	⬇️
packages/flutter_tools/lib/src/run_cold.dart	28.23% <0%> (-15.3%)	⬇️
...ckages/flutter_tools/lib/src/reporting/events.dart	84.5% <0%> (-11.27%)	⬇️
...lutter_tools/lib/src/android/android_emulator.dart	56.25% <0%> (-10.42%)	⬇️
...ges/flutter_tools/lib/src/android/android_sdk.dart	75.09% <0%> (-3.78%)	⬇️
packages/flutter_tools/lib/src/context_runner.dart	65.51% <0%> (-3.45%)	⬇️
packages/flutter_tools/lib/src/cache.dart	48.47% <0%> (-0.71%)	⬇️
... and 8 more

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