DTrace test: Add a new keyword SKIPCI to gentest.sh

[lwhsu]

Summary

This is for marking a test case is flakey and should not be executed in
the CI environment.

Sponsored by: The FreeBSD Foundation

[lwhsu]

Generated test case:

atf_test_case tst_dynopt_d
tst_dynopt_d_head()
{
    atf_set 'descr' 'DTrace test common/misc/tst.dynopt.d'
}
tst_dynopt_d_body()
{
    if [ "$(atf_config_get ci false)" = "true" ]; then atf_skip "see cddl/usr.sbin/dtrace/tests/tools/exclude.sh"; fi
    atf_check -s exit:0 -o empty -e empty \
        "$(atf_get_srcdir)/../../dtest" "$(atf_get_srcdir)/tst.dynopt.d"
}