[Test][FileCheck] Fix use of undef var

The test related to directives with prefix NUMEXPR-CONSTRAINT-NOMATCH
refers to the numeric variable UNSI which is defined by a directive with
prefix CHECK not enabled on the lit command-line. Rather than adding the
prefix CHECK to the lit command-line, this commit defined variable UNSI
in a new NUMEXPR-CONSTRAINT-NOMATCH prefixed directive. The directive
needs to contain more than just the variable otherwise the error message
from FileCheck is about the wrong line being matched.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D93346