Integrate hstox tests with toxcore Travis build

[iphydf]

This needs to do roughly:

cd .. # Get out of the toxcore clone (=> /home/travis).
git clone --recursive --depth=1 https://github.com/TokTok/hstox # Fetch hstox and its submodules.
rm -rf hstox/test/toxcore/toxcore # Remove the toxcore submodule
mv toxcore hstox/test/toxcore/ # Move the to-be-tested toxcore (from PR/branch) to the test directory.
cd hstox
make check-toxcore TEST=cov-asan # Build with coverage and address sanitizer.

This involves a bit more than just the above script:

• Install ghc (>= 7.8). If downloading and installing ghc takes too much time, put it into $CACHE_DIR (~/cache).
• Make two builds, one with BUILD=toxcore, one BUILD=hstox, and do different things on each. The toxcore build will do what the current build does, the hstox build does the above script. Both upload their coverage data.

[iphydf]

@linux-modder are you going to work on this?

[iphydf]

No activity here, so I'm doing it myself in #16.

[linux-modder]

Iphydf, I've been busy with other things too but feel free re-assigning it to yourself