[MLIR][OpenMP][NFC] Move the modifier testcases to correct directory

This patch moves the testcases from
`mlir/test/Target/LLVMIR/openmp-llvm-bad-schedule-modifier.mlir` to
`mlir/test/Dialect/OpenMP/invalid.mlir` as they test the verifier
(not the translation to LLVM IR).

Reviewed By: NimishMishra

Differential Revision: https://reviews.llvm.org/D120877