a few fixes in test.c

[Vigeant]

• added a missing include,
• allocate enough room for a null byte on the heap for msg (the first test failed on my system)
• initialize the msg heap chunk with null bytes.

[lyusupov]

I confirm that the test fails on Raspberry Pi unless this patch https://github.com/watson/libmodes/pull/4/commits/b6eab95a2536294aba9e5d7d64e01f1224eb3fcf is applied.

pi@raspberrypi:/run/tmp/libmodes $ ./tests/test  dump.bin
validating message #1
test: tests/test.c:87: test: Assertion `strcmp(msg, messages[msgNo++]) == 0' failed.
Aborted