tests: print expected duration in hwtimer_wait

EG:
```
$ time pseudoterm /dev/ttyUSB0
Using /dev/ttyUSB0 as serial device.
Port "/dev/ttyUSB0" opened at 115200 baud
Reset CPU (into user code)
RIOT MSP430 hardware initialization complete.

kernel_init(): This is RIOT! (Version: 2014.05-666-g2bdf-starfish-msp430-fix-thread-yield)
kernel_init(): jumping into first task...
This is a regression test for a race condition in hwtimer_wait.
When the race condition is hit, the timer will wait for a very very long time.
The test should take about 153 seconds.
success
SIGINT received, exiting...
pseudoterm /dev/ttyUSB0  0.00s user 0.00s system 0% cpu 2:43.83 total
```