Remove the mr from the base fixture

[vyasr]

Description

This mr is just an alias for the current memory resource, so we don't really need it. This came up in #14010 (comment). This PR removes all uses of it, but does not actually remove the mr yet. That will be done in a follow-up (see #14057 (comment)).

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[vyasr]

There's something very strange happening here. I can remove all uses of the mr from the fixture, but removing the mr itself is causing a single test failure in a test that isn't otherwise modified in this PR and does not use the mr function. Furthermore, the test only fails on arm, and only for certain types. In the interest of debugging a minimal example, I'm going to revert just that part of this PR, merge it, and then debug the issue separately (will probably need to get access to an arm machine). It seems likely that the issue is not directly related to the mr, but is some other problem perhaps caused by somewhere that we're not initializing/tearing down/otherwise interacting with GTest improperly.

[vyasr]

/merge