Using the shell to run unittests allows not needing
to wait for a string at the start of the test which
makes the test independent having the application reset
after the terminal is open. The same goes for triggering
sending UDP test pkts.
Adds a test case for when the following conditions cause a crash:
- a subsequent fragment is received before the first
- the reassembly buffer is currently filled up when another fragment of
a different datagram arrives and thus needs to be cached out to make
room for the new reassembly
While looking at tests/gnrc_ipv6_ext_frag again while writing
RIOT-OS/Release-Specs#137, I noticed that several of tests that I
definitely wrote myself from scretch are attributed wrong (and
sometimes even documented wrong). I guess this was caused by just
copy-pasting the files...
