Configuration file support (TOML) tests are missing

[jnd-au]

TOML config file support (and associated logging) was recently added to tz (#1) but it lacks unit tests:

1. It reduced HEAD unit-test coverage from 42% to 31%, and reduced coverage from 84% to 73% in PR Daylight Saving Time (DST) support #59 #62.
2. It introduced user-impacting changes of behaviour, bugs, or blockers like: Conflicting keymaps (config-related) #63 Crash: SIGSEGV: segmentation violation (config-related) #65 Regression: Missing UTC (config-related) #66

Therefore, config file parsing is a significant surface area for unit testing.

Furthermore, testing is needed for the configuration environment-variable support, but this is already included with bugfixes in PR #62.

[jnd-au]

Furthermore, the config file functions need to be parameterised to unblock unit testing.