[mlir][test] Fix -Wsign-compare in TestDialect.cpp (NFC)

llvm-project/mlir/test/lib/Dialect/Test/TestDialect.cpp:597:31:
error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int64_t' (aka 'long') [-Werror,-Wsign-compare]
  if (getVarOperands().size() != expectedNumOperands)
      ~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~
1 error generated.