tests/client-server.scm (%knownhosts): Use `abs_top_builddir'

* tests/client-server.scm (%knownhosts): Use `abs_top_builddir'
  instead of `abs_top_srcdir' to fix the permission issue on `make
  distcheck' during tests.
* tests/Makefile.am (AM_TESTS_ENVIRONMENT): Export `abs_top_builddir'.