feat: bloc_test: toPrettyString optional

**Description**

We are runinng tests in CI environment and trying to convert flutter test results into JUnit.xml format in order to display test results in CI environment (Azure DevOPS, but in Gitlab for instance it is similar).

However Tests suite JUnit.xml format using XML version 1.0 which forbids to use some special characters such as escape characters for colors.

Would be possible to configure if we want colored output from `blocTest` ?

This output of diff is problematic
```
==== diff ========================================

[SplashState.initial(), SplashState.noProfiles()[-, SplashState.noProfiles()-]]

==== end diff ====================================
````