Refine selftest for Armv7-M

[mkannwischer]

This hoists out a few tweaks to the selftest from the armv7m branch:

1. Fix a bug in the setter of slothy.config.selftest which made it impossible to disable the selftest via configuration
2. Ignore hints in the assertions of the selftest
3. Skip the selftest if symbolic registers are present
4. Fix a typo in the unicorn error handler
5. Fix the armv7m_simple0 example - it was missing annotations re dependencies through memory.

Resolves #118