"Observer child not in immediate context is not rebuilt" test added

[subzero911]

Added a test checking that Observer's child not in immediate context will not be rebuilt

related to PR https://github.com/mobxjs/mobx.dart/pull/909 , issue https://github.com/mobxjs/mobx.dart/issues/551

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Pull Request Checklist

• [x] If the changes are being made to code, ensure the version in pubspec.yaml is updated.
• [x] Increment the major/minor/patch/patch-count, depending on the complexity of change
• [x] Add the necessary unit tests to ensure the coverage does not drop
• [x] Update the Changelog to include all changes made in this PR, organized by version
• [x] Run the melo run set_version command from the root directory
• [x] Include the necessary reviewers for the PR
• [ ] Update the docs if there are any API changes or additions to functionality

[codecov[bot]]

Codecov Report

Merging #910 (f84645e) into master (d99a5c6) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #910   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines           97        97           
=========================================
  Hits            97        97           

Flag	Coverage Δ
flutter_mobx	100.00% <ø> (ø)

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[subzero911]

Wrong test, see https://github.com/mobxjs/mobx.dart/issues/911