[mlir] Add test to check if standalone dialect is registered

Summary: Add a test to check if the standalone dialect is registered within standalone-opt. Similar to the mlir-opt commandline.mlir test.

Reviewers: Kayjukh, stephenneuendorffer

Reviewed By: Kayjukh

Subscribers: mehdi_amini, rriddle, jpienaar, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, grosul1, frgossen, jurahul, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80764