Readme benchmark improvements

[rubvs]

Motivation/summary

The current version of README in testing/benchmark is repetitive and can be confusing. This PR is an attempt to try and resolve some of these redundancies by:

• Leveraging a bullet-point format
• Defining defaults and environment variable overrides
• Making it clear when to use specific make functions

[mergify]

This pull request does not have a backport label. Could you fix it @rubvs? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label.