build: drop a painfully long gnulib test

On machines with massive amounts of CPUs, the gnulib 'test-lock'
could take minutes, or even appear to deadlock, because of poor
scaling of timing interactions between multiple cores.

See https://bugzilla.redhat.com/show_bug.cgi?id=797284.
For precedence, note that iwhd has done the same:
https://lists.gnu.org/archive/html/bug-gnulib/2012-01/msg00311.html

We can re-enable things if gnulib ever analyzes and improves the
situation.

* bootstrap.conf (gnulib_tool_option_extras): Avoid lock-tests.

bootstrap.conf

@@ -174,6 +174,7 @@ gnulib_tool_option_extras="\
  --with-tests\
  --makefile-name=gnulib.mk\
  --avoid=pt_chown\
+ --avoid=lock-tests\
 "
 local_gl_dir=gnulib/local