t/op/smartkve.t: single-quote some eval stuff

Three tests were wrong because they incorrectly double- rather than
single-quoted an eval string like

    eval "keys $hash ...."

Since the tests were supposed to generate syntax errors anyway it didn't
show up. However, the $hash ref was being expanded to HASH(0x......)
and on some platform / compiler permutations it would generate some
mysterious

    Hexadecimal number > 0xffffffff non-portable

warnings.