cpp-qt5-client: Improve tests

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

This PR refactor the Qt5 client Petstore tests:

• remove unnecessary headers
• remove unnecessary constructor/destructor for test classes
• avoid QString constructor when possible
• PetStore executable returns the number of failed tests
• Simplify test logic user QTimer::singleShoot() static method
• don't declare lambda if used only one time
• use simple connect to lambda form
• replace QList<QString> by {..., ..., ...}

The idea is to reduce the test code base without altering its readability. All the change are based on my opinions and I'll be totally ok to discard change that doesn't comply with good practice you'll oppose it to! 😃

@ravinikam @stkrwork @etherealjoy @muttleyxd