[MLIR] Add unit test for tblgen Op build methods

- Initiate the unit test with a test that tests variants of build() methods
  generated for ops with variadic operands and results.
- The intent is to migrate unit .td tests in mlir/test/mlir-tblgen that check for
  generated C++ code to these unit tests which test both that the generated code
  compiles and also is functionally correct.

Differential Revision: https://reviews.llvm.org/D84074