Otherwise, the following would occur:
* settings.toml is in the process of being written by host computer
* soft-reset begins
* web workflow tries to grab CIRCUITPY_WIFI_SSID, but loops forever
because FAT filesystem is in inconsistent state and file reads error
* settings.toml write by host computer never completes and the filesystem
remains corrupt
* restarting yields a soft-bricked device, because startup reads
CIRCUITPY_WIFI_SSID again
This is a small optimization, it avoids reading the full file when an
early key is requested.
In the case of an *invalid* TOML file such as
```
K=80
K=81
```
this stops the value of K actually returned being 8081 and makes it 80
instead; but as it's a malformed file it doesn't really matter much.
* use a virtual fat filesystem during the test
* this makes the file I/O part more closely patch runtime which is nice
* side-steps the need to add a special function for testing
* but test still can't be run on a device, because the vfs calls
are incompatible, and you intentionally can't remount "/" anyway
* and side-steps problems with storing 'bad' toml files
