Adds code coverage to FormatStringEditorTests

[ricardobossan]

Related #10773

Proposed changes

• Adds code coverage to FormatStringEditor

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

Attention: Patch coverage is 88.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 75.42684%. Comparing base (e3efbcc) to head (2c00cc4). Report is 24 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #12208 +/- ## =================================================== + Coverage 75.38243% 75.42684% +0.04440% =================================================== Files 3099 3103 +4 Lines 634429 634314 -115 Branches 46874 46876 +2 =================================================== + Hits 478248 478443 +195 + Misses 152759 152438 -321 - Partials 3422 3433 +11 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/12208/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/12208/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `75.42684% <88.00000%> (+0.04440%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/12208/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `17.97520% <ø> (-0.03066%)` | :arrow_down: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/12208/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `48.80253% <ø> (+0.04842%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/12208/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `97.02496% <88.00000%> (-0.00082%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/12208/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `45.83844% <ø> (+0.04681%)` | :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.