[SystemZ][z/OS] Additional test coverage for validating dialect instructions for SystemZ

- There are certain instructions most notably those with extended mnemonics that restricted to only the gnu/att variant
- There are also certain instruction aliases/mnemonic aliases that are restricted only to the HLASM variant (see https://reviews.llvm.org/D97581, https://reviews.llvm.org/D94250 and https://reviews.llvm.org/D92185 for reference)
- This patch adds a few tests to check for the behaviour introduced in the above patches. The testing coverage could not be added in at the same time, due to parallel work being done introducing the HLASM syntax

Reviewed By: uweigand, abhina.sreeskantharajan

Differential Revision: https://reviews.llvm.org/D112172