[FlightRPC][C++] Fix flaky MacOS tests

[asfimport]

The gRPC MacOS tests have been flaking again.

Looking at grpc/grpc#20311 they may possibly have been fixed except grpc/grpc#13856 reports they haven't (in some configurations?) so I will try a few things in CI, or just disable the tests on MacOS.

Reporter: David Li / @lidavidm
Assignee: David Li / @lidavidm

Related issues:

• [C++] Sporadic Flight test crash on macOS (relates to)
• [C++] [CI] Flight test error on macOS (relates to)

PRs and other links:

• GitHub Pull Request #7298

_{Note: This issue was originally created as ARROW-8975. Please see the migration documentation for further details.}

[asfimport]

Francois Saint-Jacques / @fsaintjacques:
Issue resolved by pull request 7298
#7298