Adds code coverage to ImageIndexEditor

[ricardobossan]

Related #10773

Proposed changes

• Adds coverage for public properties and methods of the ImageIndexEditor class.

Customer Impact

• None

Regression?

• No

Risk

• Minimal

Test methodology

• Unit tests

Test environment(s)

• 9.0.100-rc.1.24452.12

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 75.44473%. Comparing base (16c923a) to head (5808153). Report is 13 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #12237 +/- ## =================================================== + Coverage 75.43346% 75.44473% +0.01127% =================================================== Files 3103 3104 +1 Lines 634314 634316 +2 Branches 46876 46876 =================================================== + Hits 478485 478558 +73 + Misses 152403 152334 -69 + Partials 3426 3424 -2 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/12237/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | Coverage Δ | | |---|---|---| | [Debug](https://app.codecov.io/gh/dotnet/winforms/pull/12237/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `75.44473% <100.00000%> (+0.01127%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/12237/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `17.98360% <ø> (-0.00008%)` | :arrow_down: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/12237/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `48.84051% <ø> (+0.02318%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/12237/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `97.02506% <100.00000%> (+0.00010%)` | :arrow_up: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/12237/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `45.87314% <ø> (+0.02414%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet#carryforward-flags-in-the-pull-request-comment) to find out more.