[test] Pass -ccc-install-dir in mac compilation db test

Pass -ccc-install-dir explicitly as the compilation database code does
not pass argv[0] to getMainExecutable(), while some systems require it
to return the correct path.  Since the relevant code is apparently only
applicable to Darwin, just pass correct -ccc-install-dir to make
the tests pass on *BSD systems.

Differential Revision: https://reviews.llvm.org/D56976

llvm-svn: 351752