[mlir][test] Require JIT support in JIT tests

A number of mlir tests `FAIL` on Solaris/sparcv9 with `Target has no JIT
support`.  This patch fixes that by mimicing `clang/test/lit.cfg.py` which
implements a `host-supports-jit` keyword for this.  The gtest-based unit
tests don't support `REQUIRES:`, so lack of support needs to be hardcoded
there.

Tested on `amd64-pc-solaris2.11` (`check-mlir` results unchanged) and
`sparcv9-sun-solaris2.11` (only one unrelated failure left).

Differential Revision: https://reviews.llvm.org/D131151