Run ARM64 unit-tests in target-specific test-runner

[wmmc88]

          Support for ARM64 tests is possibly possible in the future via target-specific test runner: https://doc.rust-lang.org/cargo/reference/config.html#target

Originally posted by Melvin Wang (@wmmc88) in #35 (comment)

[wmmc88]

This is no longer needed since we now have arm64 runners. tests are running on arm64 as of #420