feat: Probe for firmware mode before loading app

[kchr]

Description

This commit includes a helper method for checking that the device is in firmware mode before loading an application. If the check fails, a TKeyLoadError exception will be raised with a message explaining why.

It also includes test cases for the affected functionality, and adjustments to existing test cases in order for them to be compatible with probing for firmware mode in the TKey.load_app() method.

Type of change

Please tick any that are relevant to this PR and remove any that aren't.

• Bugfix (non breaking change which resolve an issue)
• Feature (non breaking change which adds functionality)
• Breaking Change (a change which would cause existing functionality to not work as expected)
• Documentation (a change to documentation)

Submission checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my changes
• I have tested and verified my changes on target
• My changes are well written and CI is passing
• I have squashed my work to relevant commits and rebased on main for linear history
• I have added a "Co-authored-by: x" if several people contributed, either pair programming or by squashing commits from different authors.
• I have updated the documentation where relevant (readme, dev.tillitis.se etc.)
• QEMU is updated to reflect changes