tests/tools_test: Use readlink() properly

Turns out that `readlink()` does not NULL-terminate the string and since
it resides on the stack we may end up with some junk:

  `/proc/self/exe point to /opt/igt/libexec/igt-gpu-tools/tools_test7<CD>\t<8A>^?`

That in turn confuses `dirname()` and we end up doing `chdir("/")`,
which explain the sporadic failures of this test where it was not able
to locate the tools.

Let's zero out the variable first and allow `readlink()` to write at
most `sizeof()-1` bytes to it, so it is always properly terminated.

Cc: Petri Latvala <petri.latvala@intel.com>
Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
Issue: #12


Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>