tests: fatal: increase coverage

- _SysFatalErrorHandler is supposed to be user-overridable.
The test case now installs its own handler to show that this
has happened properly.

- Use TC_PRINT() TC_ERROR() macros

- Since we have out own _SysFatalErrorHandler, show that
k_panic() works

- Show that _SysFatalErrorHandler gets invoked with the expected
reason code for some of the scenarios.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>