ioreg/test: Add test for #140

Write-only registers shouldn't be read from.

Also remove parentheses from test names as this upsets the snake-case
lint.
Merge pull request #148 from bgamari/ioreg-test

ioreg/test: Add test for #140
k20: Add basic watchdog support.
app_blink_k20: Init watchdog on start.
k20/watchdog: Use helper instead of inline asm.
systick: Refactoring.
app_blink_k20: Use Teensy Board LED.