[MM-1007]: Web app test cases setup and added web app test cases for some UI components

[Kshitij-Katiyar]

Summary

Webapp testcase setup and added web app test cases for the following UI components:

• Github assignee selector
• Github label selector
• Github milestone selector
• Github repo selector

[hanzei]

@rahimrahman I wonder if you have thoughts on the general setup of the test cases, given that you worked a lot of webapp test recently.

[rahimrahman]

@hanzei thanks for flagging me.

@Kshitij-Katiyar few things:

• let's use @testing-library/react instead of enzyme. Enzyme will not work with React 17+ (React 19 has just been released).
• let's eliminate the use of toMatchSnapshot() for more meaningful test.

[Kshitij-Katiyar]

@hanzei thanks for flagging me.

@Kshitij-Katiyar few things:

• let's use @testing-library/react instead of enzyme. Enzyme will not work with React 17+ (React 19 has just been released).
• let's eliminate the use of toMatchSnapshot() for more meaningful test.

@rahimrahman Sure will update the test cases with the changes

[abbas-dependable-naqvi]

@hanzei thanks for flagging me.

@Kshitij-Katiyar few things:

• let's use @testing-library/react instead of enzyme. Enzyme will not work with React 17+ (React 19 has just been released).
• let's eliminate the use of toMatchSnapshot() for more meaningful test.

Due to these changes, i don't think this PR is much useful, It would be better to build this from scratch. Still keeping this PR open for context

[mattermost-build]

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