build: avoid corrupted gnulib/tests/Makefile

build: avoid corrupted gnulib/tests/Makefile

Running 'make check' can sometimes fail in the gnulib/tests
subdirectory, when doing an incremental build, because
./bootstrap generates a Makefile.am that tries to refer to
../../.. instead of ../.., and gets lost.

This may be an upstream gnulib bug, where a more elegant
solution will present itself in the future:
http://thread.gmane.org/gmane.comp.lib.gnulib.bugs/24898

But in the meantime, I was able to reproduce both the issue,
and this solution to work around it.

* bootstrap.conf (bootstrap_epilogue): Ensure that no stray
../../.. components remain in gnulib/tests/Makefile.in.
Reported by Serge Hallyn.