Refuse to run the test suite if expect is broken.

This causes "make check" to fail quickly if running on an expect that
has broken handling of zero-lenght writes, or if expect is really,
really slow due to broken glob gate.

* src/server/testsuite/Makefile.am (check-l2g): Depend on
working-dejagnu.
(check-lyskomd): Ditto.
(check-leaks): Ditto.
(working-dejagnu): New target.
(EXTRA_DIST): Added dejagnu.0/00-zero-length-write.exp and
dejagnu.0/90-slow-glob-gate.exp.

* src/server/testsuite/dejagnu.0/00-zero-length-write.exp: New
test, that fails on known bad versions of expect.

* src/server/testsuite/dejagnu.0/90-slow-glob-gate.exp: New test,
that fails if a simple regexp pattern causes expect to run too
slow, indicating that it is converted to a glob pattern that
causes a lot of backtracking.

* src/server/testsuite/.gitignore: Ignore dejagnu.log and
dejagnu.sum.