Fix test of ses(4) when there is no SES device exists

glob(3) returns GLOB_NOMATCH if GLOB_NOCHECK or GLOB_NOMAGIC flag is not
passed so ATF_REQUIRE_EQ(r, 0) will cause a precondition check failure if no
/dev/ses* exists.

Remove calling of atf_tc_skip() in ATF_TC_CLEANUP() because it would let
the clean up procedure unfinish.

While here, fix a set-but-not-used warning.

Reviewed by: asomers
Differential Revision: https://reviews.freebsd.org/D34056