tests: Organize and label MPU unit tests

[ppannuto]

We have lots of MPU tests, and it's not necessarily obvious why.

This aims to give a "memory map" to the array of tests, as well as to make more obvious where you might start for any interactive testing.

[bradjc]

Why do we need another sub folder?

[ppannuto]

We don't need a subfolder, but I'll quote myself from the README updates for the rationale:

MPU test applications are necessarily somewhat fine-grained, as each MPU
violation results in termination of the test process.

As it can be annoying to flash myriad apps during development, the tests in the
top-level directory attempt to exercise all of the reads and writes of regions
that should work during normal operation (and allow use of a button to inject
faults for interactive testing). The unit/ folder holds all of the smaller
tests which attempt to violate access rules at each of the boundaries.

[bradjc]

But we don't have any other tests. Let's just add the folder when we know we need it. We have many groups of tests, many of which can be annoying to run. I don't think there is anything different about MPU.

[ppannuto]

Yes we do?

mpu_walk_region is still in the root folder here, and #497 adds a second root-level test.